Syrian troops backed by tanks raided houses in the city of Hama on Wednesday, searching for activists behind five months of protest against President Bashar al-Assad, residents said.
The raids took place a day after security forces killed at least four people among crowds of demonstrators pouring out of mosques in Syria after marking the end of Ramadan, the Muslim holy month in which Assad intensified a crackdown on protests.
Troops swept through several cities in Ramadan, killing scores of people and triggering Western sanctions and Arab criticism, without crushing the unrest in which the United Nations says 2,000 civilians have been killed.
The protesters have also failed to unseat Assad, but they have been encouraged by the fall of Libya's Muammar Gaddafi and rising international pressure on Syria, including a planned European Union embargo on the oil industry, which would disrupt vital inflows of foreign currency at Assad's disposal.
President Barack Obama's administration, which has already imposed sanctions on Syria's oil industry and a state-owned bank, froze the U.S. assets of Foreign Minister Walid al-Moualem and two other Syrian officials on Tuesday.
Hama has seen some of the biggest protests against 41 years of Assad family rule and was the first city assaulted during Ramadan. Authorities said the army withdrew by mid-August, but residents reported a significant military presence on Wednesday.
"Several light tanks and dozens of buses parked at al-Hadid bridge at the eastern entrance of Hama. Hundreds of troops then went on foot into al-Qusour and Hamidiya neighborhoods. The sound of gunfire is being heard," Abdelrahman, a local activist, told Reuters by phone.
"These neighborhoods have been among the most active in staging protests," he added.
Another resident said Toyota pick-up trucks mounted with machineguns and buses full of troops also assembled overnight near al-Dahiriya district at the northern entrance of Hama, 205 km (130 miles) north of the capital Damascus.
"The people want the execution of the president," a YouTube video showed dozens of protesters chanting in Hamidiya after night prayers, shortly before the raid on the district.
Most foreign media were expelled from Syria after the uprising began in March, making it hard to verify reports.
Hama was scene of a 1982 massacre by the military, sent by Assad's father Hafez al-Assad to crush an Islamist uprising.
Assad, from Syria's minority Alawite sect, has repeatedly said force is necessary to defeat what he calls a foreign plot to divide Syria, which is majority Sunni Muslim. Authorities blame armed groups for the violence and say more than 500 soldiers and police have been killed.
Activists from Syria's Kurdish minority will hold a conference in Sweden on September 3 to unify efforts against Assad, organizers said.
Massoud Akko, a Kurdish writer who lives in Norway, criticized major parties representing the Kurds, who number an estimated one million out of Syria's 20 million population.
"Kurdish politicians cannot keep issuing timid statements about the need for reform and stopping the repression as our Arab brethren keep getting massacred," he said.
Most Kurdish party chiefs say they do not want a repetition of a 2004 Kurdish revolt that Assad's forces crushed.
The Syrian Observatory for Human Rights (SOHR), based in Britain, said on Wednesday that 360 civilians had been killed during Ramadan and 113 members of the army and security forces.
State television aired an audio recording on Tuesday of what it said were two terrorists who revealed "a full agenda of provocation and targeting police and army camps and terrorizing peaceful citizens in the name of freedom and non-violence."
In the northwestern province of Idlib on the border with Turkey, troops shot dead one villager, Hazem al-Shihadi, at a checkpoint overnight near the town of Kfaruma where army defections have increased, activists said.
Around 7,000 Syrian refugees, most of whom had fled army assaults on their towns and villages in Idlib, are housed in six camps across the border in Turkey, the Euro-Mediterranean Human Rights Network said in a statement.
Syrian authorities have said that most of refugees who fled to Turkey, which put their number in excess of 10,000, had returned to their homes in Idlib after troops "cleansed" a main town in the province of "armed terrorist groups."
Residents and activists have reported a rising number of defections among Syrian armed forces, which are drawn mostly from the Sunni majority, but are dominated by Alawite officers effectively under the command of Assad's younger brother Maher.
They say most of the defectors have fled to Turkey or taken refuge in their home towns and villages, provoking raids by pro-Assad forces, and in some instances, armed clashes.