The Syrian Observatory for Human Rights, a monitoring group, reported that the suspected US-led coalition airstrike hit a Syrian army post near an Islamic State stronghold in Deir al-Zour.
Four soldiers were killed in the bombardment, according to the Britain-based Observatory.
The al-Assad regime controls most of the Deir al-Zour neighbourhoods along the western banks of the Euphrates, while Islamic State controls most of the centre of the oil-rich province.
“We know for sure that the strike was not launched by Russian planes because Moscow is an ally of al-Assad’s regime,” Observatory chief Rami Abdel-Rahman told dpa.
Russia began its aerial campaign in Syria in September with the aim of targeting Islamic State and other radical groups.
In recent weeks, the US-led alliance and Russia have separately stepped up airstrikes on Islamic State-held areas in Syria in retaliation for deadly attacks claimed by the al-Qaeda splinter group in several parts of the world.
In Rome, Italian Foreign Minister Paolo Gentiloni said his country would host a ministerial meeting of the global coalition against Islamic State next month.
No exact date has been set for the talks, which should take place in Rome in the second half of January, according to a spokesman for Gentiloni.
Major world powers have also intensified diplomatic efforts to reach a political solution to Syria’s conflict, now in its fifth year.
Saudi Arabia has said it will host this week a meeting for moderate Syrian opposition groups aimed at unifying their ranks ahead of planned peace talks between representatives of the opposition and al-Assad’s government.
There are conflicting reports about when the meeting will exactly start in the Saudi capital Riyadh. On Sunday, Saudi Arabia said the gathering would open on Tuesday and run for three days.
However, member of the opposition Syrian National Coalition, Samir al-Nashar, said the conference had been postponed to Wednesday for further preparations.
“The conference will start on Wednesday and last for a week,” al-Nashar, who will attend the meeting, told dpa.
Syria’s moderate opposition factions have been weakened by divisions, allowing jihadist rebels, mainly Islamic State, to establish a foothold in the country.
Saudi Arabia is a financial and diplomatic backer of opposition forces fighting to oust al-Assad.
The oil-rich kingdom has repeatedly called for al-Assad to step down to end his country’s multi-sided conflict, which is estimated to have claimed more than 250,000 lives.