President Bashar al-Assad has vowed to recapture the whole of Syria and keep "fighting terrorism" while also negotiating an end to the war, as international pressure mounts for a ceasefire.

His defiant stance, in an exclusive interview with AFP released Friday, doused hopes of an imminent halt to hostilities that world powers are pushing to take effect within a week.

Assad said the main aim of a Russian-backed regime offensive in Aleppo province that has prompted tens of thousands of people to flee was to cut the rebels' supply route from Turkey.

He said his government's eventual goal was to retake all of the country, large swathes of which are controlled by rebel forces or the Islamic State (IS) jihadist group.

"It makes no sense for us to say that we will give up any part," he said in the interview conducted on Thursday in Damascus.

Assad said it would be possible to "put an end to this problem in less than a year" if opposition supply routes from Turkey, Jordan and Iraq were severed.

But if not, he said, "the solution will take a long time and will incur a heavy price".

Assad said he saw a risk that Turkey and Saudi Arabia, key backers of the opposition, would intervene militarily in Syria.

World powers agreed Friday on an ambitious plan to cease hostilities in Syria within a week, but doubts soon emerged over its viability, especially because it did not include IS or Al-Qaeda's local branch.

US Secretary of State John Kerry said there were "no illusions" about the difficulty of implementing a nationwide "cessation of hostilities" as he announced the deal in Munich alongside Russian Foreign Minister Sergei Lavrov.

Lavrov underlined that "terrorist organisations" such as IS and Al-Qaeda-affiliated Al-Nusra Front "do not fall under the truce, and we and the US-led coalition will keep fighting these structures".

A Pentagon spokesman confirmed the plan will not affect operations of the US-led international coalition against IS.

- Humanitarian aid -

Moscow says its more than four-month-old bombing campaign in Syria targets IS and other "terrorists", but critics accuse Russia of focusing on mainstream rebels.

The Munich deal went further than expected, with Lavrov talking about "direct contacts between the Russian and US military" on the ground, where the powers back opposing sides in the five-year-old conflict.

However, Pentagon spokesman Captain Jeff Davis said at a press conference there would be no increased military cooperation in Syria between the US and Russia.

The 17-nation International Syria Support Group also agreed that "sustained delivery" of humanitarian aid will begin "immediately".

But after Assad's forces this month nearly encircled Aleppo, Syria's second city, several nations put the onus on Moscow to implement the deal.

"Through its military action on the side of Assad's regime, Russia had recently seriously compromised the political process. Now there is a chance to save this process," German foreign ministry spokeswoman Christiane Wirzt said.

"What is important now is embracing this opportunity, stopping the airstrikes, ceasing targeting civilians and providing humanitarian access," added Turkish Foreign Minister Mevlut Cavusoglu on Twitter.

He later said that Russian bombing killed 16 civilians in Syria early Friday.

"Despite the agreement we made last night, Russia continued bombing the civilians -- they killed 16 civilians this morning," he said in Munich.

UN Secretary General Ban Ki-moon welcomed the plan to cease hostilities in Syria.

"Tens of thousands of people there are in desperate need of life saving aid and the entire country urgently needs peace," he told a press conference in Montreal.

- Question marks -

However, analysts remained sceptical about the chances of ending a war that has killed over 260,000 people and displaced more than half the population.

"There are huge question marks," said Julien Barnes-Dacey of the European Council on Foreign Relations.

The failure to include Al-Nusra was particularly important, he said, since the group is active in Aleppo and surrounding regions, and many of the more "moderate" rebels have links with it.

"This effectively gives the green light for the Syrian government and its allies to carry on military action while paying lip service to the agreement," said Barnes-Dacey.

Other analysts said it was significant that the US and Russia had been able to strike a deal at all.

The US and Russia have "taken ownership of this now. This is important," said Michael Williams, a former UN diplomat in Lebanon and now at London's Chatham House think-tank.

"The parties, the opponents will notice this. It will put quite a bit of pressure on Assad and his regime. It's very hard for them now to walk away."

Peace talks collapsed earlier this month over the offensive on Aleppo, which has forced at least 50,000 people to flee and killed an estimated 500 people since it began on February 1.

A key Syrian opposition body, the High Negotiations Committee, said Friday it was up to rebels on the ground whether to implement the deal.

Kerry said talks between the opposition and the regime would resume as soon as possible, but warned that "what we have here are words on paper -- what we need to see in the next few days are actions on the ground".

burs/srm/hc

KERRY GROUP