Syrian government forces briefly captured four villages east of the Euphrates River in the eastern province of Deir el-Zour on Sunday after rare clashes with US-backed Kurdish-led fighters, then lost the area in a counter-offensive by the Kurdish-led force.
The area close to the border with Iraq has been the site of recent clashes between the two sides that had been focusing on fighting the Islamic State extremist group. IS had declared its caliphate in parts of Syria and Iraq.
Crossings into the east bank of the Euphrates in eastern Syria by government forces have been rare.
Hours later, Syrian state television reported air strikes it described as a "new aggression", with missiles targeting a number of military outposts in northern Syria.
Syrian TV reported early today that the missiles targeted outposts in the Hama and Aleppo countryside. It did not say who fired the missiles or whether there were any casualties or damage.
The news comes less than two weeks after a similar report of air strikes on government military installations in the central Homs region and the suburbs of Damascus. But the military later said a false alarm had set off air defence systems.
The Britain-based Syrian Observatory for Human Rights, which monitors the war in Syria through activists on the ground, reported loud explosions in the Hama countryside and Aleppo province resulting from missiles targeting a base of the 47th Brigade in Hama that houses Syrian government and allied Iranian forces.
It also reported missiles hitting positions near the Nairab air base and Aleppo airport. It said the source of the missiles was not immediately known, and had no information on casualties.
Earlier this month, seven Iranian military personnel were killed in an air strike on Syria's T4 air base in the central Homs province. Syria, Iran and Russia blamed Israel for that attack.
Israel, which has frequently targeted what it says are weapons shipments to the Lebanese militant group Hezbollah in Syria, did not confirm or deny it.
The US, Britain and France also carried out joint air strikes on Syrian government facilities on April 13, in retaliation for a suspected chemical weapons attack on a rebel-held town near Damascus they blamed on President Bashar Assad's government.
There was no immediate comment from Israel or the US.
State news agency Sana said the villages briefly captured by government forces on Sunday were held by the US-backed Syrian Democratic Forces, adding that they are close to the provincial capital, also called Deir el-Zour.
The SDF said in a statement later that it regained control of the whole area it earlier lost.
Much of Deir el-Zour province was held by the Islamic State group but over the past year Syrian government forces captured most areas west of the Euphrates while SDF fighters took areas east of the river.
On February 7, pro-Syrian government fighters attacked SDF positions east of the river and faced a ferocious US counter-attack which left dozens dead, including Russians.
SDF spokesman Kino Gabriel said in a statement earlier on Sunday that the Syrian army attack coincided with "our forces' preparations to complete the Island Storm campaign" to liberate the remaining areas east of the river from IS.
He said the Syrian army and pro-government fighters began targeting SDF fighters to impede "the launching of our campaign against terrorism. Our forces are responding in self-defence".
"We affirm that we are determined to eradicate terrorism from its roots and to assert our right to self-defence," he said of IS. "We consider this aggression by regime forces to be a support for terrorism and falls within the attempts to impede the war on terrorism."
The Observatory said Sunday's offensive left six SDF fighters dead and 22 wounded, adding that there were also casualties on the government side.
Mr Gabriel issued another statement later, saying all the area lost earlier was regained by SDF fighters. He said Syrian troops were backed by Russian fighters adding that after the SDF's counteroffensive, government forces "are now far away".
The Observatory's chief Rami Abdurrahman said SDF fighters took back most of the area except for one village.