The United States threatened to end all cooperation between the U.S. and Russia to stop Syria's civil war, unless Moscow and Bashar Assad's forces ended their deadly assault on the opposition-held eastern Aleppo. The warning was made in a phone call by Secretary of State John Kerry to his Russian counterpart Sergey Lavrov on Wednesday. The U.S. condemned the mounting loss of life in the Russian and Syrian air assaults on Aleppo, favoring a diplomatic solution that would restore the Syria cease-fire deal. It was the stiffest U.S. warning to the Russians since the Sept. 19 collapse of a truce they jointly brokered.

U.S. State Department spokesman John Kirby said that "Moscow has a decision to make," at the State Department press briefing Sept. 28 while warning Russia of "attacks on Russian cities." "Given what we have seen on the ground and in the air around Aleppo, unless something dramatically changes very, very soon … we are going to have to take those steps to suspend our bilateral engagement on Syria," Kirby said. He added "That is not an insignificant move for us."

Deputy Foreign Minister Sergey Ryabkov said on Thursday that the U.S. statements on the suspension of U.S.-Russia dialogue amounted to blackmail. "It's a policy of threats and blackmail aimed at imposing solutions favorable to the U.S. and its clients," Ryabkov told RIA Novosti.

Addressing the U.S.'s tough response to Russia over the Syria crisis, Russian Defense Ministry spokesman Igor Konashenkov stated on Thursday that Russia will continue U.S.-Russia dialogue so long as the threats to Russia are ruled out. "We once again state that we are fully ready for continuing a dialog with the American side and for continuing work on joint measures in the fight against terrorists in Syria," he said. "However, even signs of threats to our military and Russian nationals must be ruled out in this dialog. There will be no bargaining in the issues of ensuring the safety of Russian nationals no matter where they stay. This is our main and over-riding priority," Konashenkov said.

Obama administration officials have begun considering tougher responses to the Russian-backed Syrian government assault on Aleppo, including military options. Tensions continue to rise with Moscow diminishing hopes for diplomatic solutions from the Middle East to Ukraine and cyberspace, U.S. officials said on Wednesday.

The new discussions were being held at "staff level," and have yet to produce any recommendations to President Barack Obama, who has resisted ordering military action against Bashar Assad in the country's multi-sided civil war. The U.S. officials said the failure of diplomacy in Syria has left the Obama administration no choice but to consider alternatives, most of which involve some use of force and have been examined before, but held in abeyance. The heavy use of Russian airpower in Syria has compounded U.S. distrust of Russian President Vladimir Putin's geopolitical intentions, not only in the five-and-a-half-year civil war, but also in the Ukraine conflict and in what U.S. officials say are Russian-backed cyber-attacks on U.S. political targets.

President Barack Obama acknowledged Wednesday that he would not deploy a game-changing force in Syria to end the brutal fighting since U.S. national security was not at stake. Obama's comments came at a town hall conversation hosted by CNN, at a military base in Fort Lee, Virginia.

"There are going to be challenges around the world that happen that don't directly touch on our security," the American leader said. "It is in these situations where you have to make judgments about what is best for the national security interests of the United States, even though what you see is heartbreaking."

Addressing the criticism, Obama said that as Assad was supported by Russia and Iran, the U.S. would have to deploy large numbers of troops and take a risk of confrontation with Russia.

He also said that the use of military force would have been a violation of international law as the Assad regime was not a direct threat to the U.S.

Russia is militarily involved in Syria, supports separatists in eastern Ukraine and areas of Georgia and has even been accused of trying to meddle in the U.S. presidential race. At the same time, the Obama administration has been forced to accept that working with Russia is probably the only way to achieve results on many complicated international issues. Russia was central in the Iran nuclear negotiations and is a player as well as negotiator in the Syria truce effort.

Meanwhile, unidentified jets on Thursday bombarded bakeries in two rebel-held areas in Syria's northern province of Aleppo, activists said. "One bakery was bombed in the town of Anadan in the northern countryside of Aleppo and a second was struck in the town of Kfar Naha in the western part of Aleppo countryside," said a local activist, who gave his name only as Wissam for security reasons. At least three people were injured in the air raids on Kfar Naha, Wissam added.

On Wednesday, 11 people were killed in government shelling that hit a bread queue in opposition-controlled eastern Aleppo, according to activists.

Airstrikes by the Syrian government and allied Russia have killed more than 298 people in eastern Aleppo since Sept. 19, when a ceasefire brokered by the United States and Russia collapsed, according to the Observatory.