Israel has pledged to prevent its main enemy from entrenching itself militarily in Syria, where it is backing President Bashar al-Assad's government alongside Russia and Lebanese militant group Hezbollah.

The pre-dawn raids killed seven foreign fighters in the Kisweh area south of Damascus, the Syrian Observatory for Human Rights said.

Iranian and pro-Iran foreign fighters were present in the area, the Britain-based war monitor said, but the nationality of the dead was not immediately clear.

Five Syrian members of a pro-Iranian group were also killed in the Ezra area of the southern province of Daraa, Observatory head Rami Abdel Rahman said.

A Syrian army source quoted by state news agency SANA said air defences responded to two waves of Israeli strikes after midnight that targeted the Damascus area and then positions in Daraa and the adjacent province of Quneitra.

"The attack wounded eight fighters," the source said, without elaborating on where they had been stationed or their nationality.

AFP correspondents in Damascus heard loud explosions around 1:15 am (2315 GMT Wedensday).

State television broadcast images showing explosions in the sky.

An Israeli army spokesman declined to comment on the strikes when contacted by AFP.

Israel has carried out repeated strikes in Syria since the civil war erupted in 2011, mainly targeting government forces and their Iranian and Hezbollah allies.

Israel's political leadership has spoken publicly of the bombing camaign, although the army rarely comments on individual strikes.

- 'Will not save' Idlib -

Last month, Damascus accused the Israeli air force of carrying out an attack on the T4 airbase in central Syria, which the Observatory said killed at least three Iran-backed militiamen.

In December, the Observatory said Israeli air strikes killed three foreigners fighting alongside government forces south of the capital.

The previous month, the Israeli army claimed responsibility for a wave of air strikes against Syrian military sites and Iranian forces that killed 23 people including 16 foreigners.

Israeli Prime Minister Benjamin Netanyahu said positions of the Iranian Revolutionaty Guards foreign operations arm were among the targets.

The war in Syria has killed more than 380,000 people and displaced millions since it started in 2011 with the brutal repression of pro-democracy protests.

The reported Israeli strikes come as government forces press a blistering offensive against Syria's last major rebel bastion, in the Idlib region in the northwest.

The Syrian army source said the strikes would not deter government forces from retaking the region, which is dominated by jihadists of Syria's former Al-Qaeda affiliate.

"This escalation will not save the armed terrorist groups that are collapsing in Idlib and western Aleppo under the strikes of the Syrian Arab Army," the source said.

The bombardment has killed around 300 civilians since mid-December, the Observatory says.

The violence has seen more than 500,000 civilians flee their homes over the past two months, the United Nations says.