Islamic State (IS) has poured more fighters into Ramadi as security forces and Shi'ite paramilitaries renewed efforts to retake the western Iraqi city that fell to the Islamists a week ago in a major setback for the government.

In Palmyra, the Syrian air force launched strikes at buildings captured by IS whose occupation of the city has raised fears that the insurgents will destroy its famed Roman ruins.

IS has killed at least 217 people execution-style, including children, since it moved into the Palmyra area 10 days ago, according to the British-based Syrian Observatory for Human Rights. A further 300 soldiers were killed in fighting before the city was captured, according to the monitoring group.

IS has reinforced Ramadi, deploying fighters in preparation for battle against security forces and paramilitary groups advancing on the provincial capital.

Iraqi forces have regained ground east of the city since launching a counter-offensive on Saturday, a week after it was overrun by the insurgents, and also retook a rural area south of the city.

Ramadi residents said trucks carrying IS fighters arrived on Sunday evening before spreading out across the city.

The fall of Ramadi is the most significant setback for Iraqi forces in almost a year and has cast doubt on the effectiveness of a US-led strategy for fighting the Sunni militant group.

The seizures of Ramadi and Palmyra were the group's biggest successes since a US-led coalition launched an air war against it last year.

The near simultaneous victories against the Iraqi and Syrian armies have forced Washington to examine its strategy, which involves bombing from the air but leaving fighting on the ground to local forces.

The general in charge of Iran's paramilitary activities in the Middle East meanwhile said the US and other powers were failing to confront IS.

"Today, in the fight against this dangerous phenomenon, nobody is present except Iran," said Major General Qassem Soleimani, who is often seen on the battlefields of Iraq.