MUNICH (AP) — Senior Republican Sen. John McCain sharply criticized the deal to seek a temporary truce in Syria, arguing Sunday that Russia is engaging in "diplomacy in the service of military aggression."

Diplomats from a group of countries that have interests in Syria's five-year civil war, including the U.S., Russia, Turkey, Saudi Arabia and Iran, agreed on Friday to seek a temporary "cessation of hostilities" within a week. They also agreed to "accelerate and expand" deliveries of humanitarian aid to besieged Syrian communities beginning this week.

It remains unclear whether those commitments can be made to stick on the ground and whether deep differences regarding the truce and which groups would be eligible for it — between the U.S. and Russia among others — can be overcome.

The truce deal in Munich comes as Syrian government forces, aided by a Russian bombing campaign, are trying to encircle rebels in Aleppo, the country's largest city, and cut off their supply route to Turkey.

"I wish I could share the views of some of my friends who see this agreement as a potential breakthrough but unfortunately I do not," McCain, who chairs the Senate Armed Services Committee, said at the Munich Security Conference.

"Let's be clear about what this agreement does: it permits the assault on Aleppo to continue for another week. It requires opposition groups to stop fighting, but it allows Russia to continue bombing terrorists — which it insists is everyone, even civilians," he said.

"If Russia or the (President Bashar) Assad regime violates this agreement, what are the consequences?" he asked. "I don't see any."

McCain said that Russian President Vladimir Putin "is not interested in being our partner. He wants to shore up the Assad regime. He wants to re-establish Russia as a major power in the Middle East."

"This is diplomacy in the service of military aggression and it's working because we are letting it," he said.

The head of the six-nation Gulf Cooperation Council, which includes key rebel backers Saudi Arabia and Qatar, offered a more upbeat assessment. He said the agreement to cease hostilities is a "signal of hope" and was optimistic that it could be achieved.

"It is an opportunity for us to turn our undivided attention on Daesh, which is probably the single most challenging global threat," said GCC Secretary General Abdullatif al-Zayani, using the Arabic acronym for the Islamic State group. "This is a chance to work together on an issue that unites and doesn't divide."

He added that it was also an opportunity to deliver humanitarian aid desperately needed by Syrian civilians.

McCain and al-Zayani were speaking on the final day of the three-day Munich conference, an annual gathering of foreign and security policy leaders.

Earlier Sunday, Somalia's president said military forces have made inroads in their fight against al-Shabab militants but the key to defeating them lies in providing better opportunities for the country's youth.

President Hassan Sheikh Mohamud said 70 percent of his country's population is under 35 and have grown up in the chaos of a lawless state after the government collapsed in 1991. That, he said, has left them "very, very vulnerable to be recruited by the evil forces" like al-Qaida-linked al-Shabab.

Mohamud said many of those fighting for al-Shabab "are not there for ideological reasons, they are there for economic reasons ... to feed their families."

Somalia faces regular al-Shabab attacks even after driving them from the capital, Mogadishu, in 2011.

David Rising contributed to this story.