Syria: US warns Damascus over protest killings
The US has pledged a "strong international response" against Syria's government if Damascus does not end a brutal crackdown against protesters.
At least 21 people were killed in Homs, Hama and other cities, reports said, as Friday saw another day of violence.
The US called the violence "deplorable" and said it would take "additional steps" if President Bashar al-Assad did not take steps to end the bloodshed.
More than 500 people are thought to have been killed since mid-March.
President Bashar al-Assad's government has said it is taking decisive action against terrorists and criminals.
In Friday's violence, human rights activists said at least 15 people died in the central city of Homs, with six others killed in Hama.
Ten members of the Syrian security forces were also killed, the government said, blaming "terrorists".
Reacting to the violence, the US said it strongly condemned the Syrian government's use of violence and mass arrests.
"We again salute the courage of Syrian protesters for insisting on their right to express themselves and we regret the loss of life on all sides," White House spokesman Jay Carney said.
The US backed European Union plans to impose travel bans and asset freezes on at least 13 senior Syrian figures for their part in the violent crackdown, which has also seen at least 2,500 people arrested.
The sanctions will come into force later this month after being formally adopted by heads of government.
But the White House spokesman said Damascus still needed to make "significant changes" to its approach to the unrest or risk further, unspecified, action from by Washington.
The situation required "an end to the government's killing of protesters and to the arrest and harassment campaigns of protesters and activists, coupled with a genuine political reform process responsive to the demands of the Syrian people," Mr Carney said.
Meanwhile, the UN said Syria had agreed to allow its humanitarian teams into the country.
Troops and tanks have withdrawn from the southern city of Deraa, where a human rights group says troops carried out a 10-day "massacre".
Stones and bullets
Protesters had vowed to make Friday a "day of defiance", and thousands clashed with security forces outside a mosque in central Damascus after Friday prayers.
Reliable sources also told the BBC there were protests in Darbasiya, Amouda and Zabadani, as well as in towns near Deraa, a city not far from the border with Jordan which has been a focus of the unrest.
Demonstrations were also reported in the Damascus suburb of Saqba, the town of Tal, north of the capital, and the coastal town of Baniyas.
In Tal, witnesses told Reuters news agency that security forces had fired at protesters.
"Five bodies were picked up in the Bab al-Sibaa area [of Homs]. There are scores of injured protesters," one told the Reuters news agency.
Ten Syrian soldiers and policemen were killed in Homs by "terrorist" groups, said a military spokesman quoted by the state news agency, Sana.
None of these reports can be verified independently, as foreign journalists are not allowed into Syria.
Prominent dissident Riyad Sayf - who has spent years in prison since President Bashar al-Assad came to power - was also arrested, activists said.
Earlier, troops - including tanks - were deployed in a number of cities and towns in anticipation of renewed protests.
Numerous checkpoints were set up in Damascus and elsewhere, witnesses say. A heavy troop presence was reported in Homs, nearby Rastan, and Baniyas.
Hundreds of families were said to be fleeing Baniyas, fearing the city - like Deraa - could come under siege.
"It looks like they are preparing to attack the town, like they did in Deraa," one activist told the AFP news agency by telephone from the town.
Across Syria, protesters have been calling for greater political rights and personal freedoms. Some are calling for the downfall of the regime.
The unrest in Syria poses the most serious challenge to Mr Assad since he succeeded his father, Hafez, in 2000.
Foreign journalists are not allowed to enter the country, so it is difficult to verify the reports of deaths.
But one doctor, who planned to join those demonstrating, said the "indiscriminate killings and inhumane arrests have generated total disgust among the average Syrian".
"Soldiers with rifles no longer deter people. The propaganda that this regime is the only guarantor of stability no longer washes," he was quoted as telling the Reuters news agency.
A Red Cross team has arrived in Deraa carrying medical supplies and humanitarian aid.
The head of the Damascus office of the International Committee of the Red Cross (ICRC) said Deraa "is a priority for us, because it is the city that has been hardest hit by the ongoing violence".
A mobile phone snapshot, reportedly taken in Qamishli on 29 April, shows protesters carrying banners written in Arabic and Kurdish demanding democracy.
Razan, who is a resident of Damascus, tells the BBC about violence and protests around Syria.
This unverified video seems to show a peaceful protest in Talbisah. Moments into the footage, tanks fire on unarmed civilians. Wyre Davis reports.
Residents of Deraa walk past a burnt-out building. It follows shelling by troops in what human rights groups say was an intensified crackdown on protests in recent weeks.
Syrian army vehicles were photographed near Homs and broadcast on the Syrian opposition internet channel Sham SNN on 11 May.