Just two months after the United States and Russia joined together to forge a partial cease-fire in Syria, the alliance between the two countries barely “hangs by a thread,” according to the U.N. envoy to Syria.

Russia is accusing the Obama administration of bending to its regional allies by ignoring the presence of terrorists among opposition forces in the fight against Syrian President Bashar Assad.

A Foreign Ministry spokesman sarcastically commented on President Obama’s recent decision to send in another 250 Special Operations troops to fight Islamic State terrorists in Syria, The Washington Post reported. Obama had previously pitched a scenario where there would be no American “boots on the ground.”

The Foreign Ministry spokesman asked if the U.S. was instead planning to deploy troops barefoot.

The recent comments reflect the rising tension between the U.S. and Russia.

The Washington Post reported Thursday there is “growing dissension over whether to call Russia out for acting in bad faith.”

Earlier Thursday, the United Nation’s envoy for Syria appealed to both countries to intervene and help revive stalled peace talks, saying a recent spike in fighting had overshadowed talks and put an increasingly feeble truce in “great danger.”

Staffan de Mistura told reporters that the cessation of hostilities agreed upon by the Western- and Saudi-backed opposition and envoys from Assad’s government – backed by Moscow –“hangs by a thread.”

"I really fear that the erosion of the cessation is unraveling the fragile consensus around a political solution, carefully built over the last year," de Mistura said in his U.N. Security Council briefing. “Now I see parties reverting to the langue of a military solution or military option. We must ensure they do not see that as a solution or an option.”

De Mistura said he’s urging the leaders of the U.S. and Russia to intervene because both presidents’ legacies are “linked to the success of what has been a unique initiative which started very well.”

U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov largely spearheaded the current effort to end the five-year Syrian conflict.

De Mistura appealed not only for U.S.-Russia intervention but for international support, calling in the Security Council briefing for another ministerial meeting of the International Syria Support Group "lest we lose the window of opportunity to reverse the negative downward spiral."

The International Syria Support Group includes 17 countries as well as the United Nations, Arab League, European Union and Organization of Islamic Cooperation.

De Mistura said he hoped that the talks would resume in May, and he predicted the overall process would continue as previously planned through July. But he stopped short of setting a specific date, pointing to the recent surge in fighting, notably in and around Aleppo.

According to Reuters, the opposition group – High Negotiations Committee – pulled out of formal talks a week ago to protest  the intensified fighting and the slow response for aid deliveries.

Bashar Ja’afari, who led the government delegation, said Tuesday the latest round of talks was “useful and constructive,” but gave no sign of giving in to the opposition’s demands of forming a new government without President Assad.

Lamenting the worsening violence, he said that "in the last 48 hours, we have had an average of one Syrian killed every 25 minutes, one Syrian wounded every 13 minutes ... How can you have substantial talks when you have only news about bombing and shelling?"

Speaking at the end of a third session of Syria peace talks this year, de Mistura said the truce brokered by the U.S. and Russia had pulled off a "miracle" by sharply reducing violence in March, but acknowledged that the renewed fighting has put the cessation of hostilities "in great danger."

He called for a "U.S.-Russian initiative at the highest level" to help reinforce it.

"There is no reason that both of them — who have been putting so much political capital in that success story and have a common interest in not seeing Syria ending up in another cycle of war — should not be able to revitalize what they created, and which is still alive, but barely," he said of the two countries.

State Department spokesman John Kirby said he shared de Mistura’s concerns and urged Russia to press the Assad regime “to fulfill its commitments.”

De Mistura suggested back-channel work between Moscow and Washington was already under way to help the truce, and that a revival of it would help bring the two sides back to the table.

"I know that both the Russian Federation and the U.S. are talking among themselves on how to salvage on what has been actually a remarkable success — but needs to be sustained," he said, referring to the cease-fire. If it is revived, he predicted: "It will not be difficult for everyone to come back around the table."

The Associated Press and Reuters contributed to this report.