Russian airstrikes in southern Syria hit an anti-Islamic State rebel force backed by the United States, news reports quoted the US government as saying Friday.
US broadcaster CNN quoting US officials said that Russian planes carried out airstrikes on Thursday near the Iraqi and Jordanian border against the New Syrian Army (NSA). The 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 President Bashar 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 news comes amid fresh signs of dissent within the US diplomatic service over how to handle the situation in Syria.
A "dissent channel cable" was signed by 51 State Department officers involved with advising on Syria policy calling for 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. 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.
"The Wall Street Journal reviewed a copy of the cable, which repeatedly calls for targeted military strikes against the Syrian government in light of the near collapse of the ceasefire brokered earlier this year," the report said.
Russia and the US have been pushing for ceasefire, 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.
Despite the near total collapse of the ceasefires, one advantage of the US-Russia-brokered deals has been increased aid deliveries to vulnerable areas.
However, the Syrian government remains accused of being the primary party responsible for blocking aid.