The United Nations aid convoy delivering food was hit by air strikes which pounded opposition-held parts of Syria on Monday night, minutes after the Bashar al-Assad regime unilaterally declared the end of a ceasefire. The strikes that killed more than a dozen people, carried out by either Syrian or allied Russian warplanes, hit Syrian Red Crescent trucks as staff members were unloading aid in Urem al-Kubra in northern Aleppo province.

The Russian Defense Ministry denied claims on Tuesday saying that neither Russian nor Syrian war planes had struck a humanitarian convoy near Aleppo the previous day, stating the convoy's whereabouts had only been known to militants on the ground.

The Syrian Red Crescent said the head of one of its local offices and "around 20 civilians" were killed in the attack. The U.S. State Department said it was "outraged" by the bombing and would be raising the matter directly with Russia. Igor Konashenkov, a spokesman for the Russian military, denied any Russian involvement in the attack in a statement read out on the state-run Rossiya 24 TV channel on Tuesday.

"All information on the whereabouts of the convoy was available only to the militants controlling these areas," Konashenkov said. That appeared to contradict a statement by U.N. humanitarian aid spokesman Jens Laerke, who told reporters in Geneva earlier on Tuesday, that Russia had been notified about the convoy in advance.

Kremlin spokesman Dmitry Peskov earlier on Tuesday made a series of gloomy comments on Syria, saying the situation was "a source of great concern" and that there was little hope for a renewal of a ceasefire there. The Assad regime also denied claims that it was behind airstrikes that hit an aid convoy in northern Syria.

The Assad regime is responsible for the bulk of the violence inflicted on the Syrian people in that country's civil war, said the U.N. Secretary General Ban Ki-moon during his speech at the U.N. General Assembly. "Many groups have killed many innocents – but none more so than the government of Syria, which continues to barrel bomb neighborhoods and systematically torture thousands of detainees," Ban said. In his last speech before the assembly as secretary general, Ban said that "powerful patrons that keep feeding the war machine also have blood on their hands."

"Present in this hall today are representatives of governments that have ignored, facilitated, funded, participated in or even planned and carried out atrocities inflicted by all sides of the Syria conflict against Syrian civilians," he said.

U.N. suspends Syria aid deliveries

The U.N. humanitarian aid agency suspended all convoys in Syria on Tuesday following deadly airstrikes on aid trucks the previous night that activists said killed at least 12 people, mostly truck drivers and Red Crescent workers.

In Geneva, spokesman Jens Laerke of OCHA said on Tuesday that further aid delivery would hold pending a review of the security situation in Syria in the aftermath of the airstrikes. Laerke called it "a very, very dark day... for humanitarians across the world."

U.N. officials said the U.N. and Red Crescent convoy was delivering assistance for 78,000 people in the town of Uram al-Kubra, west of the city of Aleppo. Initial estimates indicate that about 18 of the 31 trucks in the convoy were hit, as well as the Red Crescent warehouse in the area.

U. N. Humanitarian Chief Stephen O'Brien called on "all parties to the conflict, once again, to take all necessary measures to protect humanitarian actors, civilians, and civilian infrastructure as required by international humanitarian law."

Also Tuesday, the Observatory said Assad's forces launched an offensive in the Handarat area, just north of the city of Aleppo, in what appears to be an attempt to tighten the siege on opposition-held parts of Syria's second largest city. Apart from the 12 killed in the convoy attack, 22 civilians died in attacks Monday across the province, according to the Observatory and Aleppo 24 News. Since the shaky truce started on Sept. 12, the opposition reported 254 violations by Assad's forces and their allies, a senior Syrian opposition official declared the cease-fire "clinically dead."

U.S., Russia-led renewed Syria talks began in New York

The U.S. Secretary of State John Kerry and Russia's Foreign Minister Sergei Lavrov opened a meeting of their key international counterparts on Tuesday after a week-old ceasefire in Syria's civil war collapsed. The 23-nation International Syria Support Group (ISSG) gathered in a New York hotel amid bitter recriminations between Moscow and Washington over the failure of an agreement to enforce the truce.

"The United States is outraged by reports that a humanitarian aid convoy was bombed near Aleppo today," State Department spokesman John Kirby said.

Last week, the United States was forced to apologize after it weakened the ceasefire by bombing Syrian troops, but Washington said it had been accidental.

U.S. officials said there could be no similar excuse from Russia for the targeting of non-combatant aid workers.

The Syria ceasefire came under added strain over the weekend when the U.S.-led coalition jets against DAESH killed more than 90 Assad fighters in the eastern Syrian city of Deir al-Zor. Deir al-Zor attack has clearly led to more deteriorated relationship between U.S. and Russia over Syria truce. The U.S. expressed its "regret" over the incident, which has raised tension with Damascus's chief ally Moscow. Together with Australia and Denmark, Britain said on Monday it had taken part in U.S-led air strikes in Syria over the weekend that Russia says killed Assad forces.

The U.S.-Russian agreement marks the second ceasefire negotiated by Washington and Moscow this year in the hope of advancing a political solution towards ending the war, now in its sixth year that has killed hundreds of thousands of people.