AMMAN: Syrian forces spread through southern towns Thursday and tightened their grip on two other cities, broadening a crackdown ahead of what could prove a pivotal day for protests against President Bashar Assad.

While Assad has promised reforms in the hope of dampening dissent, tanks advanced in the southern towns of Dael, Tafas, Jassem and Al-Harra before Friday – the Muslim day of prayer which has become the main day of protests across the Arab world.

Friday prayers offer the only chance for Syrians to assemble in large numbers, making it easier to hold demonstrations. This week will be a particularly important test after the government said it had largely put down the unrest.

Tanks were deployed in areas on the Syrian coast, the central region of Homs, outside the city of Hama to the north and now across the southern Hauran Plain, regions which cover large swathes of the country of 20 million people.

The official SANA news agency said army units were chasing “armed terrorist groups,” backed by Islamists and foreign agitators, whom authorities have blamed for the violence. The government says about 100 soldiers and police have been killed, including two on Wednesday in the cities of Homs and Daraa.

Foreign journalists have been barred from the country, making independent accounts difficult to obtain.

The Geneva-based International Commission of Jurists said reports it had received from lawyers and rights groups in the country described attacks on civilians that amounted to crimes under international law.

“More than 700 people have reportedly been unlawfully killed and hundreds subjected to enforced disappearances,” it said. “The ICJ continues to receive credible reports that bodies have been left in the streets for days and the injured blocked from accessing medical facilities.”

U.S. Secretary of State Hillary Clinton said Washington and its allies would hold Assad’s government to account for “brutal reprisals” against protesters and may tighten sanctions, but she stopped short of saying Assad should leave power.

Washington and its European allies have been criticized for a tepid response to the violence in Syria, in contrast with Libya where they are carrying out a bombing campaign they say will not end until leader Moammar Gadhafi is driven from power.

The United States and Europe so far have imposed economic sanctions on a handful of senior Syrian officials, not including Assad himself.

“President Assad faces increasing isolation and we will continue to work with our international partners in the EU and elsewhere on additional steps to hold Syria accountable for its gross human rights abuses,” Clinton said.

Asked if Assad had lost his legitimacy to rule, she demurred but said Washington has watched with “great consternation and concern as events have unfolded under his leadership.”

A prominent lawyer in the southern Hauran region, where the uprising erupted in March, said hundreds of people had been arrested in the region since Wednesday, when a rights activist said 13 people were killed by tank shelling in the area. There was no immediate comment from Syrian authorities.

In the besieged coastal city of Banias and nearby village of Baida, security forces arrested scores of residents Thursday, two Syrian human rights organizations said.

“The sound of heavy gunfire was heard as security forces made the arrests,” a spokesman for the Syrian Observatory for Human Rights said.

In Homs, security forces arrested a veteran human rights campaigner, Naji Tayara, Thursday, the group said. Tayara had been an outspoken critic of a military incursion into the city’s residential neighborhoods.A main residential neighborhood in Homs remained sealed by security forces after it was shelled by tanks Wednesday and at least five people were killed, a witness said.

“I passed by a major road block at the main entrance to Homs off the highway to Damascus. Armed security men were checking names and they asked me what business I had going into Homs,” a woman who travelled to Homs from Damascus to see relatives said.

Assad has responded to the unrest with promises of reform, lifting a 48-year-old state of emergency and granting stateless Kurds Syrian citizenship last month. Rights groups say thousands have been arrested and beaten since he made the promises.

The 45-year-old president, who had been emerging from Western isolation before the unrest and strengthening ties with NATO member Turkey, has reinforced an alliance with Iran.

Turkish Prime Minister Tayyip Erdogan criticized Syria’s use of force, saying this week: “it’s not an armed group you’re firing at … it’s just people in this case.”

Protests have continued for nearly eight weeks, but the two main cities of Damascus and Aleppo have not seen major unrest.

The head of Israel’s Shin Bet said Syria would be “soaked in blood”, because the ruling Alawite Shiite minority sect was “fighting for its life.”