An international tribunal will hand down a ruling on Tuesday in an increasingly bitter dispute over the South China Sea, in a closely-watched case that risks ratcheting up tensions in Southeast Asia.

The Permanent Court of Arbitration (PCA) -- the world's oldest international arbitration tribunal -- will issue a written decision at 11:00 am (0900 GMT) after the Philippines challenged China's claim over much of the strategic waterway.

Manila lodged the suit against Beijing in 2013, saying after 17 years of negotiations it had exhausted all other political and diplomatic avenues.

Angered by the move, Beijing refused to participate, adding it would not comply with the ruling by a tribunal with "no jurisdiction" over the issue.

Legal experts agree that after three years of deliberations, two hearings and nearly 4,000 pages of evidence, the court in The Hague is likely to find in Manila's favour -- in a decision with far-reaching ramifications.

"An award from the tribunal that rejects some of China's more dubious claims would provide support for the mainstream views of other states in the region," Cecily Rose, assistant law professor of Public International Law at Leiden University, told AFP.

"China is bound to comply with the award.

"But should it refuse to do so, the tribunal has no enforcement mechanism to which it can turn," Rose said.

- Military tensions -

The judgement comes against the backdrop of frequent military brushes between China and its Asian neighbours the Philippines, Vietnam, Malaysia, Brunei and Taiwan, which ring the waters believed to hold untapped oil and gas reserves.

The tensions have also alarmed the United States which has key defence treaties with many regional allies, and in a show of strength last week sent warships to patrol close to some of the reefs and islands claimed by China.

Washington on Friday "urged both parties to comply with the ruling and urge all claimants to avoid provocative actions or statements".

"Whatever the outcome, the case will not contribute to improved relations between China and other claimants in the South China Sea," Frans-Paul van der Putten, a senior researcher at the Clingendael think-tank, told AFP.

Newly-elected Philippine President Rodrigo Duterte said he was optimistic of a favourable ruling, but offered to hold conciliatory talks with China. And he vowed he would not "taunt or flaunt" any favourable ruling.

Philippine Foreign Secretary Perfecto Yasay, also in a softening of Manila's stance, said Friday the country was willing to share natural resources with China.

But China's state-run media said Friday Beijing will not take "a single step back" in the waterway.

The Asian economic and military powerhouse claims most of the South China Sea, even waters approaching neighbouring countries, as its sovereign territory, basing its arguments on Chinese maps dating back to the 1940s marked with a so-called "nine-dash line".

Expert Joris Larik from The Hague Institute for Global Justice think-tank pointed out that Manila is aiming to "pierce" the nine-dash line, adding: "China's position and credibility will be weakened if the arbitral panel finds against it."

But Chinese President Xi Jinping said Beijing was "not afraid of trouble" and state-run media urged China to prepare for "military confrontation" in the waters.

China launched drills in the sea on Friday, state media reported, with the navy carrying out combat exercises with live missiles according to the PLA Daily, the military's official newspaper.

- No 'historic' claim -

Manila stresses it is not asking the court to resolve sovereignty or delimit boundaries, but is seeking decisions on some key points.

Referring to the UN Convention on the Law of the Sea, Manila contends the "nine-dash" line has no basis under international law and Beijing has no "historic" claim to the waters.

Manila has further argued that Beijing violated the convention by preventing it from exercising its fishing and exploration rights and that Chinese fishing boats have irreversibly damaged coral reefs.

The Philippine government also says some of the "islands" claimed by Beijing are merely rocks, and not entitled to a "territorial sea" or an exclusive economic zone.

Tensions between China and its neighbours in the sea have at times flared into open conflict including in 1988 when Vietnam and China fought a naval battle on Johnson Reef in the Spratly Islands which killed about 70 Vietnamese.

The sea's shipping lanes connect East Asia with Europe and the Middle East, and over $5 trillion (4.5 trillion euros) in ship-borne trade passes through them annually.

South China Seas case lights up world's oldest tribunal
The Hague (AFP) July 10, 2016 - The little-known Permanent Court of Arbitration on Tuesday rules on a bitter dispute over the South China Sea which could have huge international ramifications amid growing tensions in the key waterway.

The Philippines brought the case against the China in 2013, asking the court to find that Beijing's claims to much of the territory in the sea are invalid and violate the UN's Convention on the Law of the Sea.

Here are five facts about the tribunal based in The Hague:

What is the PCA?

The PCA is the world's oldest inter-governmental organisation dedicated to resolving international disputes through arbitration "and other peaceful means".

It came to life in 1899 during the first Hague Peace Conference convened by Czar Nicholas II of Russia. It refers to contracts, special agreements and various treaties such as those set up by the UN Commission on International Trade Law (UNCITRAL) and the UN Convention on the Law of the Sea (UNCLOS) to rule in disputes.

It also has a permanent overseas presence in Mauritius and can conduct hearings all over the world.

Interesting PCA cases

The PCA's arbitral tribunals have rendered more than 70 decisions in past cases and it currently considers 116 cases. Recently-concluded cases include rulings in a bitter border dispute between Eritrea and Ethiopia and handing down an award in favour of the Indian Ocean nation of Mauritius in a fight with Britain over a marine protected area in the Chagos Archipelago. In another case, it gave India a partial green light to build a hydro-electric project in the Kishenganga River after a dispute with Pakistan, who was worried about the project's impact on water supply further downstream.

Is it a real 'court'?

The PCA is not a court in the traditional sense with judges to rule on issues. Rather it consists of arbitral tribunals put together for each case. Hearings are not open to the public or press, unless both parties in the dispute agree.

How does it work?

When diplomacy fails between two states they may turn to arbitration via the PCA. Usually cases are settled on a pre-existing agreement -- contained in a treaty or contract -- that if a dispute arises it will be resolved through arbitration. Once arbitration begins, an arbitral tribunal is appointed, consisting of one, three or five members. For the South China Sea arbitration a five-member panel has been appointed led by Ghanian-born judge Thomas A. Mensah.

Are its decisions binding?

Yes. All decisions, called "awards" are binding on all the parties in the dispute and have to be carried out without delay. There are some post-award proceedings available to parties unhappy with the tribunal's decision, but they are limited, particularly in inter-state disputes. Experts also say enforcement is often the "Achilles Heel" of public international law. However, states who ignore or disregard the PCA's ruling risk losing credibility and losing out in the so-called "court of world opinion".

Source: PCA staff, PCA website, and Joris Larik, senior researcher at The Hague Institute for Global Justice.