Regime change in Syria could plunge the region into chaos, a Kremlin spokesman warned on Friday, a day after media reports that dozens of US diplomatic staff members had called for military strikes against Syrian President Bashar al-Assad.
It was the latest sign of growing tensions between Moscow and Washington. The news comes on top of reported Russian airstrikes against a US-backed Syrian rebel force that has focused on fighting Islamic State.
"Liquidating a regime will hardly facilitate successful progress in the fight against terrorism. It can make the region plunge (into) total chaos," Dmitry Peskov, a Kremlin spokesman, told reporters according to the TASS news agency.
The comments seemed to be in response to a "dissent channel cable" signed by 51 State Department officers in which they advise a Syria policy of military strikes against al-Assad's government, the Wall Street Journal and The New York Times reported
The diplomats, of various ranks, argued that the current policy in Syria was benefiting al-Assad. There was no sign President Barack Obama intends to change course in Syria, where the administration is focused on defeating Islamic State.
US officials admit al-Assad is stronger than he was a year ago, largely thanks to Russian air power backing his forces on the ground.
Earlier, US broadcaster CNN quoted US officials saying Russian planes carried out airstrikes on Thursday near the Iraqi and Jordanian border against the New Syrian Army (NSA).
The rebel group is based near the al-Tanf border crossing, which it seized from Islamic State.
"The planes carried out two strikes, one that hit inside the NSA camp and the other at its outskirts killing two soldiers from NSA - one was a Syrian and the other Iraqi - as well as wounding four others," the Britain-based Syrian Observatory for Human Rights said in a separate statement, without identifying the source of the strike.
The NSA, backed by the US and trained by Jordan, was designed as a force to combat the Islamic State extremist group and is comprised of rebel fighters, many hailing from the Free Syrian Army, the main moderate force that is battling al-Assad.
"We are aware of reports that moderate Syrian opposition were bombed today in southern Syria and we don't know all the details, but, if indeed that action was taken by the Russians, we would have serious concerns about that," Pentagon Press Secretary Peter Cook said in a briefing on Thursday.
The Wall Street Journal said the cable calling for a tougher line against al-Assad, five years into the civil war, came "in light of the near collapse of the ceasefire brokered earlier this year."
Russia and the US have been pushing for ceasefires and brokering deals on the ground since February, though Russia is accused of breaching the cessation of hostilities itself a number of times, by launching airstrikes against rebel positions.
Some US State Department officials are known to differ with President Barack Obama, who has steered clear of direct military intervention against al-Assad, while granting various types of support to some rebel groups.
Hillary Clinton, the former secretary of state and now presumptive Democratic Party nominee for president, had argued a hawkish line in favour of giving more arms to rebels and further aiding their cause.
Meanwhile, a humanitarian aid convoy was able to reach the besieged al-Waer area in central Syria, a UN official confirmed, saying the area has 37,500 residents.
A second convoy is planned in the coming days, pending government approval, according to the official.
Despite the near total collapse of the ceasefire, which did hold for a period, one advantage of the US-Russia-brokered deals has been increased aid deliveries to vulnerable areas.
Since the beginning of the year, 844,325 people in hard-to-reach and besieged areas received humanitarian assistance, the UN says.
However, the Syrian government remains accused of being the primary party responsible for blocking aid.