Washington, Moscow- U.S. sources indicated that a phone conversation between U.S. Secretary of State John Kerry and his Russian counterpart Sergei Lavrov on Friday was marred with tension and resentment in light of recriminations between the two sides against the backdrop of intensifying bombardments on Aleppo, north Syria, detaining civilians and hampering the delivery of humanitarian aids.

The one hour phone call, according to sources close to the matter, addressed means of reviving agreement on cessation of hostilities.

The two parties also confirmed the importance of maintaining Syrian institutions and providing opportunities for diplomatic efforts in order to negotiate on all key issues to achieve political settlement in Syria.

On the other hand, Russia expressed its resentment on what U.S. State Department Spokesman John Kirby said as he warned Russia of ‘more body bags” and “attacks on Russian cities” if the Russians will not cooperate with them in Syria.

Kirby, said in a press conference on Thursday: “We are working through steps that we might have to begin to suspend our engagement with Russia on Syria. We haven’t taken those steps yet.”

“The message to the Russian Foreign Minister today was that we are perfectly willing and able to move forward on those steps that would end with the suspension of U.S.-Russia bilateral engagement in Syria.”

Early on Wednesday, U.S. Secretary of State John Kerry threatened to end all cooperation between the United States and Russia to stop Syria’s civil war, unless Moscow and Damascus ended the current attack on East Aleppo.

In this matter, Deputy Secretary of State Antony Blinken said: “we also are actively considering other options” at Obama’s direction for ending the civil war and he would share them with Congress once the deliberations are complete.

Kerry said on Thursday: “It’s one of those moments where we’re going to have to pursue other alternatives for a period of time barring some other clearer indication by warring parties that they’re considering how to approach this more effectively.”

Moreover, analysts indicated that there is no Plan B for the United States in Syria.

As members of the Senate Foreign Relations Committee demanded to know what “Plan B” in Syria is, Blinken described the conflict as a trap that Russia doesn’t want to get caught in.

Blinken said Russia “has a profound incentive” to find ways out. Russia escalated its involvement in Syria’s civil war because it feared losing its only foothold in the Middle East, he said.

But if the war escalates and more weaponry pours into the country, Russia will be left propping up Assad in an increasingly smaller part of Syria. A bad situation, he said, will have gotten worse for Russia.