The Syrian government has launched attacks on rebel-held areas around several cities, including Homs and Damascus suburb, according to opposition activists and a monitor.
One person was killed in the Damascus suburb of Douma and three in the al-Waer neighbourhood of Homs in Sunday's air strikes, said the Syrian Observatory for Human Rights (SOHR), while shells and rockets were used in Deraa and Idlib provinces.
"Today's escalation began in the early afternoon with repeated air strikes," said Bebars al-Talawy, an activist based in al-Waer.
SOHR, a Britain-based monitor, said that in addition to the air strikes, al-Waer was being shelled.
The development came a day after the government-held part of Homs was subjected to multiple suicide attacks.
Also on Sunday, according to Syrian state media and monitors, the Syrian government forces seized the town of Tadef from the Islamic State of Iraq and the Levant (ISIL) group on the outskirts of the northern city of Al Bab that is already under the control of Turkish-backed rebels.
"We will continue war on terrorist organisations, whatever their names, until security and stability are restored to every inch of the Syrian lands," the army said in a statement, according to the state news agency SABA.
SOHR reported that the government forces and allies entered Tadef just hours after ISIL, also known as ISIS, withdrew from there.
By taking ISIL territory south of Al Bab, the forces of President Bashar al-Assad is preventing any possible move by Turkey and the rebel groups it supports to expand southwards, and is moving closer to regaining control of water supplies for Aleppo.
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Earlier this month, a senior Russian official said Tadef marked an agreed dividing line between the Syrian army and the Turkey-backed forces.
The eastward advance south of Tadef has extended Syrian army control across 14 villages and brought it within 25km of Lake Assad, the stretch of the Euphrates above the Tabqa dam.
The Syrian army and its allies also made a new advance against ISIL around Palmyra, coming to within a few miles of the ancient desert city that the group recaptured in December, according to SOHR and the government-controlled Syrian Central Military Media.
The SOHR said the government now controls hills that oversee three oilfields west of Palmyra.
Elsewhere in Syria, Jabhat Fatah al-Sham, a rebel group previously known as al-Nusra Front, killed and injured dozens of government forces on Sunday in an attack near the Lebanese border, according to SOHR.
The attack by Levant Liberation Body - part of al-Sham - targeted government positions in the northwestern part of rural Damascus, the SOHR said.
Founded in January, the Levant Liberation Body is headed by Jabhat Fatah al-Sham and includes four other groups.
Source: News agencies