Relief efforts are being ramped up for two famine-struck regions of Somalia controlled by a US-designated terrorist group, but there are deep concerns as to the implications of dealing with them.

With the Al-Qaeda-inspired Shebab maintaining a ban on several foreign aid groups -- despite saying earlier this month they had lifted restrictions -- key UN and international agencies will be likely forced to work through local partners.

That however is far from simple: despite aid agency assurances, some observers fear the aid could still bolster support for the rebels.

Food aid delivered into areas they control still risks adding "legitimacy to them," said Stig Jarle Hansen, a Somalia specialist and associate professor at Norway's University of Life Sciences.

The hardline Islamists have waged a bloody campaign to overthrow the country's Western-backed government -- including abducting a minister on Thursday.

Many say it was the Shebab's blockade of aid since 2009 that turned widespread drought across the region into famine in areas they control.

With the Shebab controlling forces on the ground, even third-party dealings to work with them to deliver aid remains controversial.

"This is so preposterous an idea it is barely believable," said John O'Shea, head of the Irish-based aid agency GOAL, which is providing food to Somali refugees.

"None of the warmongers who have helped bring this disaster about, and are currently exacerbating its effects, should be involved in the delivery of aid," he said.

But up to 350,000 people in famine hit areas face starvation if aid is not urgently provided, the UN warns, with 12 million struggling from drought across the Horn of Africa.

But others point out the Shebab has been weakened due to pressure by community leaders angry with the crisis.

"They (the Shebab) have lost a lot of credibility and public support," said Rashid Abdi from the International Crisis Group thinktank.

"The idea that they will somehow ride this crisis to regain public credibility is a long shot," he added.

With the ban still in place, ensuring that aid will reach those most in need is another problem.

"So few foreigners can be on the ground in Somalia due to the security situation that it will be a big problem to ensure the aid is delivered correctly," said Hansen.

Of especial concern is that the Shebab will levy taxes from contractors delivering aid to boost their coffers.

"We need to test whether they are honest in their intentions or being cynical," said Abdi.

But donors insist that aid will go where it is needed, and rebels will not be paid off.

"This is not complicated -- aid will go where the humanitarian workers can gain access," said Susan Rice, the US ambassador to the United Nations.

Nearly half of Somalia's estimated 10 million people face a food crisis and malnutrition rates are the highest in the world.

"Neither the United States nor others in the international community are prepared to pay bribes or taxes to Al-Shebab while it starves its own people," Rice added.

International aid has got through: the UN children's agency last week airlifted the first supplies into Shebab-controlled areas, saying the delivery to Baidoa town "went well".

"We use independent third party monitoring to make sure that the aid reaches to those in need," said Shantha Bloemen, a UNICEF spokeswoman.

The UN food agency also insists aid will not benefit the gunmen.

"Those in control of various parts of the south are not one controlling command," said Emilia Casella, spokeswoman for the World Food Programme.

"It's important to note that we're working where we can. We're making plans to work where it's feasible."

But there are also those who feel that engagement with the isolated Shebab could provide potential advantages.

"This is a crisis, an absolute crisis, but one that could have positive political implications for the long term," said Abdi.

"Any engagement (with the Shebab) is good -- opening up a dialogue can help draw out the pragmatists and moderates within them."