North Korea fired short-range Scud missiles off its east coast for the second time in a week, drawing condemnation from Washington and Seoul as the allies held joint military exercises.

South Korea called Monday's missile launches a "reckless provocation" while the United States demanded the North show restraint and abide by UN Security Council resolutions.

The two Scud missiles "flew in a northeasterly direction and landed in the sea," Pentagon spokesman Lieutenant Colonel Damien Pickart told AFP.

The United States is "closely monitoring the situation on the Korean peninsula," he said.

The missile tests have clearly been timed to coincide with annual South Korea-US military exercises that started a week ago and run until mid-April.

The two missiles flew around 500 kilometres (310 miles) into the Sea of Japan, according to South Korea's Defence Ministry.

Four short-range Scud missiles were fired in similar fashion on Thursday.

Both tests were denounced by Seoul, which urged the North to cease all testing immediately and said it would consider calling for sanctions.

"The North is taking a double-faced stance by making conciliatory gestures on one hand and pushing ahead with reckless provocation on the other," said Defence Ministry spokesman Kim Min-Seok.

The Scuds are at the longer edge of the short-range spectrum, with an estimated reach of 300-800 kilometres -- capable of striking any target in the South.

Washington said the missile tests breached UN resolutions that require Pyongyang to abandon its ballistic missile programme.

"We urge North Korea to refrain from provocative actions that aggravate tensions and instead focus on fulfilling its international obligations and commitments," including UN resolutions, said State Department spokeswoman Jen Psaki.

The North must "exercise restraint and take steps to improve its relations with its neighbours," she added.

It is not unusual for North Korea to carry out such tests, which often go unreported by South Korea.

But South Korea said the Scud firings were of particular concern.

- 'Show of force' -

"We believe that the North is testing various ballistic missiles with various ranges as a show of force to threaten us," the defence ministry spokesman Kim said.

Pyongyang routinely condemns the South-US joint exercises as rehearsals for invasion.

Last year the drills coincided with a sharp and unusually protracted surge in military tensions that saw North Korea issuing apocalyptic threats of pre-emptive nuclear strikes.

By contrast, this year's drills began as relations between Seoul and Pyongyang were enjoying something of a thaw.

They overlapped with the end of the first reunion for more than three years of families divided by the Korean War -- an event that raised hopes of greater cross-border cooperation.

Pyongyang had initially insisted that the joint exercises be postponed until after the reunions finished. But Seoul refused and -- in a rare concession -- the North allowed the family gatherings on its territory to go ahead as scheduled.

Most analysts believe the missile tests reflect Pyongyang's need to flex its muscles in the wake of the reunion compromise.

Last week also saw an incursion by a North Korean patrol boat across the disputed Yellow Sea border that has been the scene of brief but bloody naval clashes in the past.

No shots were fired and the vessel retreated to its side of the boundary after repeated warnings from the South Korean navy.

From Monday, South Korea launched a two-day live-fire exercise involving an army artillery, naval ships and jet fighters to test their readiness against a North Korean incursion off the east coast, the South's defence ministry said.