Thirteen bodies have been retrieved from a mass grave in Deraa, the hub of Syria's protest movement, according to residents cited by human rights organisations.

People from the southern city say hundreds of people are unaccounted for since a crackdown on protests began on 18 March and intensified when the army moved in on 25 April to try to quash unrest against Bashar al-Assad's 11-year rule.

Radwan Ziadeh, the US-based head of the Damascus Centre for Human Rights, said so far seven bodies had been identified by residents. Five of the dead were reportedly from the same family: Abdulrazaq Abdulaziz Abazied and his children, Samer, Samir, Suleiman and Mohamed.

Wissam Tarif of the rights group Insan said: "Nobody knows who is behind the grave but the fact that there were people with hands tied behind their back and we have seen an operation across the country by the army … makes us believe the state is behind this."

The National Organisation for Human Rights in Syria also reported a grave found on Monday. "Authorities immediately cordoned off the area to prevent residents from recovering the bodies," it said. Several videos purporting to show the unearthing of bodies from a field near the city have been posted online.

The government said reports of a mass grave were "completely untrue", state TV reported on Tuesday.

Accounts of the mass grave could not be independently verified, although the pro-regime newspaper al-Watan has acknowledged that five bodies had been found.

"Given that Syria's officials have demonstrated time and time again that they are incapable of launching an independent investigation, it should be the UN-mandated international inquiry that looks into these killings," Nadim Houry, the Beirut-based senior researcher at Human Rights Watch, told the Guardian.

A one-day nationwide general strike was called by protesters for Wednesday, in a strategy aimed at undermining the regime's authority in Syria, embarrassing Assad, and demonstrating support for the uprising in places such as central Damascus, where big protests have yet to take place.

"It will be a day of punishment for the regime from the free revolutionaries," said a statement posted on Facebook.

The official Syrian news agency, Sana, said Assad met a delegation from Deraa and they discussed the "positive atmosphere there as a result of co-operation between the residents and the army".

Residents report that landlines have been restored, the curfew shortened and tanks have retreated to the outskirts, but the city remains under tight control.

Nick Harvey, Britain's armed forces minister, said it was "highly likely" the international criminal court would seek the arrest of Assad for his role in the crackdown on protesters.

Britain has demanded the end to the use of violence but has not called for him to step down.

Meanwhile the US condemned Syria's role in the breaching of the Israeli border by protesters on Nakba Day, and the EU and US this week will consider further moves, including sanctions on Assad, the Palestnians' name for the anniversary of the founding of Israel.

Thousands of protesters marched through the town of Saqba close to Damascus on Monday night for the funeral of Ahmed Ataya who died from wounds sustained at a protest last month, while at least 15 tanks were deployed around Arida, near the border town of Tel Kelak.

Activists said at least seven civilians were killed in Tel Kelak on Sunday when troops shelled the town, and one was killed on Monday, raising the death toll since troops entered on Saturday to 12. Syrian officials say five soldiers were killed by armed gangs in the town.

Veteran dissidents who have met government officials say that officials acknowledge the protests in private.

But in public, authorities have blamed most of the violence on armed groups backed by extremists and foreign powers.

"Syria's leaders talk about a war against terrorists, but what we see on the ground is a war against ordinary Syrians," Sarah Leah Whitson, Middle East director of Human Rights Watch, said in a statement on Sunday.

Nidaa Hassan is a pseudonym for a journalist in Syria