U.S. President Barack Obama warned that the United States may walk away from a Syrian cease-fire deal reached with Russia last week unless Syria allows humanitarian aid to reach needy people.

After Obama met with his national security team on September 16, the White House issued a statement saying Obama "expressed deep concern that, despite decreased violence across the country, the Syrian regime continues to block the flow of critical humanitarian aid." .

Obama "emphasized that the United States will not proceed with the next steps in the arrangement with Russia until we see seven continuous days of reduced violence and sustained humanitarian access," the White House said.

The president's statement came after U.S. Secretary of State John Kerry told his Russian counterpart Sergei Lavrov that Washington will not agree to begin joint targeting of Islamic militants in Syria, as called for after seven days of cease-fire under the deal, until Syria permits the aid to flow.

State Department spokesman John Kirby said Kerry told Lavrov by telephone that the United States expects Moscow to use its influence on Syrian President Bashar al-Assad "to allow UN humanitarian convoys to reach Aleppo and other areas in need."

"Repeated" delays in allowing the convoys the aid to enter the country have been "unacceptable," Kerry said.

The ultimatum to Russia and Syria came at the same time the United States and Russia abruptly cancelled a scheduled United Nations Security Council meeting on the Syrian cease-fire deal on September 16 and as signs emerged that the truce that went into effect on September 12 was fraying.

After three days which saw a marked decrease in violence and no deaths, the first civilians since the start of the truce were killed on September 15, according to the Syrian Observatory for Human Rights, a monitoring body. Three more died and 13 were injured in air strikes in rebel-held Idlib province on September 16, the Observatory said.

Shells were also fired by insurgents into two besieged Shi'ite villages and clashes hit areas east of Damascus as well on September 16.

But the biggest problem under the deal has been the Syrian government's refusal to let UN aid into rebel-held areas of Aleppo that are surrounded by pro-government forces and where an estimated 300,000 civilians are trapped without food and other necessities.

The UN has blamed the government for holding up its aid convoys by denying them letters guaranteeing access.

Another provision of the deal faltered on September 16 when Syrian government forces returned to their positions along the Castello Road, where they previously had withdrawn under an agreement to clear the way for civilians and aid to move alongthe key thoroughfare.

Viktor Poznikhir, the first deputy chief of the Russian General Staff's operations directorate, said that Syrian troops returned their tanks, armored vehicles, and artillery to their original positions because they were being shelled by opposition groups.

While he said Russia is ready to extend the cease-fire for another 72 hours if the United States agrees, Poznikhir returned some rhetorical fire at Washington, warning that the situation on the ground "could get out of control" unless the United States forces rebels it backs to comply with the cease-fire and clear the vital roadway.

He said the Syrian army has fully complied with the truce, while opposition forces have violated it 144 times .

"We expect decisive measures from the American side, aimed at influencing the armed groups under their control to rigorously carry out the September 9 agreement," he said.

Sharp disagreements between the United States and Russia also emerged over the decision to cancel the UN meeting on the cease-fire deal on September 16.

Moscow's UN ambassador charged that the meeting was cancelled because the United States was unwilling to provide security council members with documents detailing provisions of the cease-fire deal.

"We believe that we cannot ask them to support documents which they haven't seen," Ambassador Vitaly Churkin said.

The U.S. mission to the UN said it called off the meeting because it could not agree with Russia on a way to brief the council that would "not compromise the operational security of the arrangement."

"We believe the Security Council can play an important role in the resolution of the crisis in Syria," a U.S. spokesman said.

"However, right now we are focused on the implementation of the agreement...particularly the urgent need for humanitarian aid to reach Syrians in need."

Despite the exchange of accusations, Churkin said Kerry and Lavrov will hold several meetings in New York next week on the sidelines of the UN General Assembly session, one of them as co-chairs of the International Syria Support Group.

With reporting by AP, Reuters, TASS, AFP, and dpa

