BEIRUT - Jihadists have killed almost 50 Syrian regime fighters in 48 hours, a monitor said Saturday, in some of the deadliest attacks on pro-Damascus forces in recent weeks.

Kurdish-led forces in March announced the defeat of the Islamic State group's 'caliphate' in eastern Syria, but the jihadists have retained hideouts there and in other parts of the country as well as the ability to carry out deadly assaults.

Since Thursday, IS jihadists have killed 35 pro-Damascus fighters in regime-held parts of central and eastern Syria, the Syrian Observatory for Human Rights said.

Observatory chief Rami Abdel Rahman described those attacks as producing 'the highest death toll among regime forces since the caliphate was declared defeated' in the eastern village of Baghouz last month.

Meanwhile, regime fighters also came under attack on another front of Syria's grinding eight-year war, the Britain-based monitor added.

On Saturday, jihadists linked to Syria's former Al-Qaeda affiliate attacked loyalist checkpoints and positions on the western edges of the northern city of Aleppo, killing 13 more pro-Assad fighters, it said.

It was the latest death tally in a civil war that has killed more than 370,000 people since starting in 2011 with the brutal repression of anti-government protests.

President Bashar al-Assad has managed to claw back around 60 percent of the country with Russian military backing since 2015, but several regions remain beyond its control.

IS assaults

These include a large swathe of northeastern Syria held by Kurdish-led forces who have been fighting IS with the support of a US-led coalition, as well as the jihadist-held region of Idlib west of Aleppo city.

Regime forces in theory control the vast desert that stretches from the capital Damascus to the Iraqi border, but IS is still present there.

Attacks by the jihadists have killed 27 troops and allied militiamen, including four senior Syrian army officers, in the desert east of Homs province since Thursday, the Observatory said.

The propaganda arm of IS said its fighters carried out the operation.

The Observatory also said IS fighters killed another eight soldiers and militiamen, including two officers, in the eastern province of Deir Ezzor on Thursday night.

That attack targeted a desert village south of the city of Mayadeen on the Euphrates River, upstream from the village of Baghouz where IS made a desperate last stand in March.

IS swept across a large swathe of Syria and Iraq in 2014, declaring a cross-border 'caliphate' in areas it controlled.

At the height of its rule, the extremist group imposed its brutal ideology on millions in territory roughly the size of the United Kingdom.

But after various military operations in both countries, IS lost the last shred of its proto-state on the eastern banks of the Euphrates last month.

The US-led coalition has however warned repeatedly that the jihadists' loss of their last piece of territory does not mean their elimination as a fighting force.

Earlier this month, IS claimed responsibility for a twin attack on April 9 in the northern city of Raqa held by Kurdish-led forces, which the Observatory said killed 13 people, mostly civilians.

Aleppo attack

In the north of Syria, regime forces face another jihadist group.

Syria's former Al-Qaeda affiliate, Hayat Tahrir al-Sham, control the northwestern region of Idlib, which remains beyond regime control.

Idlib is supposed to be protected from a massive regime military offensive by a deal inked in September by government ally Russia and rebel backer Turkey.

But the accord has unwound as HTS took full control of the region in January, and the area has come under increasing bombardment.

The Observatory said Saturday's attack by the HTS-linked Abu Bakr Al-Sadeeq Army on the western edges of Aleppo city came after regime bombardment overnight hit eastern and southeastern parts of the Idlib region.

A planned buffer zone around the region was never fully implemented as jihadists refused to withdraw from it.

On Friday, Assad insisted the main aim of the Idlib deal was to 'eliminate terrorists', after they failed to pull out from the planned demilitarized cordon.

He urged progress on removing 'obstacles' to the stalled deal ahead of talks to be attended by Russia, fellow ally Iran, and Turkey next week in Kazakhstan.

Endless rounds of U.N.-backed peace talks have failed to end Syria's war, and the parallel track backed by Moscow, Tehran and Ankara has gained momentum.