The United States has suspended negotiations with Russia on reviving a failed ceasefire in Syria amid heightened tensions over the embattled city of Aleppo.
"The United States is suspending its participation in bilateral channels with Russia that were established to sustain the cessation of hostilities," US State Department spokesman John Kirby said in a statement on Monday.
"This is not a decision that was taken lightly," the official added.
Kirby also accused Moscow of not living up to its commitments under a ceasefire agreement by stepping up airstrikes in Aleppo.
Russia has been supporting the Syrian government in its push to take back Aleppo from the terrorists.
Syrian government forces, backed by the Russian air campaign, have managed to gain more ground against foreign-sponsored militants in the strategic northwestern city.
Secretary of State John Kerry said on Thursday that the US was "on the verge of suspending the discussion because it's irrational in the context of the kind of bombing taking place."
Russian Deputy Foreign Minister Sergei Ryabkov said the US threat to suspend talks on Syria constituted "an emotional breakdown."
Tensions have been running high between the US and Russia since last Monday’s deadly strike on a humanitarian aid convoy near the city of Aleppo. Both sides blame each other for the attack.
The US has been working with Russia for months to try to secure a ceasefire in Syria.
Moscow and Washington agreed on a milestone deal after some 13 hours of marathon talks in the Swiss city of Geneva on September 9. The truce went into effect at sunset on September 12.
The truce, however, collapsed a few days later after the Syrian government accused foreign-backed militants of repeatedly violating the cessation of hostilities.
Damascus refused to extend the deal after US-led air raids on its military base near the eastern city of Dayr al-Zawr killed 83 Syrian soldiers.
US officials have indicated that the failure of diplomacy has left the White House no choice by to consider alternatives, including some use of military force, in Syria.