(CNN) Russia has announced it is sending its only aircraft carrier to waters off Syria's coast, as diplomats met at the United Nations in an effort to revive Syria's failing ceasefire.

Russian Defense Minister Sergey Shoigu said the Admiral Kuznetsov, carrying dozens of military aircraft, would be sent to the eastern Mediterranean to join other Russian ships off the war-torn country's coast, state news agencies reported.

The announcement -- a potential contingency plan for the failure of the ceasefire -- came in the wake of surging violence, including the deadly bombing of a Syrian Arab Red Crescent aid convoy on Monday night.

Speaking at the United Nations, US Secretary of State John Kerry said the the ceasefire was "hanging by a thread." He denounced the "outrageous" attack on the aid convoy, and said the international community had been "woefully inadequate" in Syria.

Kerry said that all war planes over Syria should be grounded, to "prevent Syria from doing what it so often does in the past -- to target civilians."

UN chief: 'A make or break moment'

Speaking before Kerry, Russian Foreign Minister Sergey Lavrov said the United States had not done enough to rein in Syrian rebels under its influence, adding that Syrian government troop withdrawals had not been matched with rebel retreats.

UN Secretary-General Ban Ki-moon called for the ceasefire to be restored to enable humanitarian relief to be delivered. "We are at a make or break moment," he said.

Violence returns

Following days of relative calm with the ceasefire, deadly violence once again plagues Syria.

About 20 people were killed in Monday night's deadly strike on the aid convoy in Urum al-Kubra that prompted the United Nations to halt its aid operations in Syria.

CNN Map

The attack came two days after warplanes from the US-led coalition mistakenly killed dozens of Syrian government troops in a strike intended to target ISIS.

In the latest violence, four medical workers were killed Tuesday night in an airstrike on a facility southwest of Aleppo, according to a relief organization.

The attack occurred at a medical facility in Khan Touman, a village in Aleppo province near Urum al-Kubra, according to Zedoun Alzoubi, CEO of the Paris-based International Union of Medical Care and Relief Organizations, which runs the facility.

Nine patients at the facility were killed, Alzoubi said, adding he did not know whether they were civilians or fighters. The UK-based Syrian Observatory for Human Rights reported that the nine patients were rebel fighters.

Alzoubi said he believed the airstrike intentionally targeted the medical facility -- a charge that has repeatedly been leveled at the Syrian military and their Russian allies throughout the conflict.

ISIS shoots down Syrian warplane

In further violence, a Syrian warplane was shot down Wednesday during a combat mission against ISIS in the eastern Qalamoun area near the capital, Damascus, but the pilot was rescued, Syrian state media reported, quoting a military source.

The terror group claimed responsibility for downing the jet in a statement from the ISIS-affiliated Amaq news agency.

On Sunday, ISIS shot down another Syrian jet in the eastern province of Deir Ezzor.

US: Russia to blame for aid convoy assault

The United States has reached the preliminary conclusion that Russian warplanes bombed the aid convoy , US officials have told CNN, and say that Russia is responsible, whether it was Russian planes -- or the Syrian regime's -- that struck.

Russia denies it was responsible and says that terrorists carried out the attack, saying that analysis of drone footage of the strike showed that militants were following the convoy.

Syria strongly denies its forces were behind the attack.

The UN's humanitarian office, OCHA, is ready to resume humanitarian convoys in Syria, said spokesperson Jens Laerke.

"The preparation for these convoys has now resumed and we are ready to deliver aid to besieged and hard-to-reach areas as soon as possible," OCHA said in a statement.

What about the ceasefire?

Monday night's attack came just hours after Syrian authorities declared an end to the fragile ceasefire, which began September 12.

Soon afterward, Syrian warplanes resumed airstrikes in Aleppo, according to the Syrian Observatory for Human Rights.

While the violence appears to have left the hard-fought ceasefire in jeopardy -- US officials said they believe the agreement is still in place.

JUST WATCHED The struggle for a ceasefire in Syria Replay More Videos ... MUST WATCH The struggle for a ceasefire in Syria 01:46

Ben Rhodes, deputy national security adviser, said the US preference is to continue with the ceasefire effort, paving the way for more cooperation between the Russian and US militaries in Syria, but that Moscow's actions could prevent further coordination.

"We have not seen good faith. This was an outrageous action," Rhodes told CNN's Michelle Kosinski. "It raises serious questions about whether or not this agreement moves forward."