Iran is playing an increasingly active role in helping the Syrian regime in its crackdown on pro-democracy protesters, according to western diplomatic sources in Damascus.

The claim came as Syria's security forces backed by tanks intensified operations to suppress unrest in three new flashpoint towns on Sunday and it was confirmed that four women had been shot dead in the first use of force against an all-female demonstration.

A senior western diplomat in Damascus expanded on assertions, first made by White House officials last month, that Iran is advising president Bashar al-Assad's government on how to crush dissent.

The diplomat pointed to a "significant" increase in the number of Iranian personnel in Syria since protests began in mid-March. Mass arrests in door-to-door raids, similar to those that helped to crush Iran's "green revolution" in 2009, have been stepped up in the past week.

Human rights groups suggest more than 7,000 people have been detained since the uprising began. More than 800 people are said to have died, up to 50 during last Friday's "day of defiance". Last night two unarmed demonstrators were reportedly killed during a night rally in the eastern city of Deir al-Zor.

"Tehran has upped the level of technical support and personnel support from the Iranian Republican Guard to strengthen Syria's ability to deal with protesters," the diplomat said, adding that the few hundred personnel were not involved in any physical operations. "Since the start of the uprising, the Iranian regime has been worried about losing its most important ally in the Arab world and important conduit for weapons to Hezbollah [in Lebanon]," the diplomat said.

Last month White House officials made similar allegations about Iranian assistance for the regime, particularly in terms of intercepting or blocking internet, mobile phone and social media communications between the protesters and the outside world. But the officials did not provide hard evidence to support their claims.

Activists and diplomats claim Iran's assistance includes help to monitor internet communications such as Skype, widely used by a network of activists, methods of crowd control, and providing equipment such as batons and riot police helmets.

Syria has denied seeking or receiving assistance from Iran to put down the unrest. In a statement issued on Friday, Iran's foreign ministry stressed Syria's "prime role" in opposing Israel and the US, and urged opposing forces in the country to compromise on political reform. US policy towards Syria was based on "opportunism in support of the Zionist regime's avarice", it said.

The Assad family, from the Shia Muslim minority Alawite sect, is likely to be nervous about appearing to be helped by its Shia-dominated ally to crush protesters drawn from the 75% Sunni population.

Regime forces backed by tanks were in action over the weekend in Homs, in the town of Tafas north of Deraa, and in the coastal city of Banias, activists said. Violence was also reported in the Damascus dormitory town of Zabadani.

Along with arbitrary detentions, shootings have continued.

Razan Zeitouneh, a lawyer in the capital who is monitoring the protests, said four women were shot dead in the village of Merqeb, close to Banias, and six men were shot dead in Banias on Saturday.