MOSCOW (Reuters) - Russia is "seriously concerned" by Syrian reports that a coalition led by the United States launched air strikes on Syrian government forces in the Deir al-Zor area, the Russian Foreign Ministry said in a statement on Tuesday.

Both Russia and a U.S.-led coalition are fighting against Islamic State in Syria but, while Russia supports Syrian President Bashar al-Assad, Western countries in the coalition say he has lost legitimacy.

"Moscow is seriously concerned over reports that on Dec. 6 the U.S.-led coalition made an air strike on Syrian governmental positions in the Deir al-Zor area," the ministry said.

The ministry said it was also concerned about "the consequences of an air strike on erroneous targets... in Hasakah province that led to a significant number of casualties among civilians".

On Monday, Syria's government said coalition planes had carried out a deadly air strike on the Syrian camp. The United States denied the allegation and a U.S. military official, speaking on condition of anonymity, said the United States was certain that Russia was responsible. Russia has also been striking targets in the Deir al-Zor area.

"In general, these cases indicate that the situation in the areas of fighting with so-called Islamic State in Syria and Iraq is heating up," the Russian Foreign Ministry said.

The ministry also said it considered the presence of Turkish forces in Iraq near Mosul illegal and a serious source of tension.

(Reporting by Alexander Winning; Writing by Katya Golubkova; Editing by Kevin Liffey)