As the Daraa crackdown raged into a second day, Britain said it was working with its partners to send a "strong signal" to Damascus, and France and Italy denounced the "unacceptable" situation in Syria.
Amnesty International also called for the UN Security Council to refer the situation in Syria to the International Criminal Court.
Washington ordered non-essential embassy staff out of Syria, which has been shaken by six weeks of protests against President Bashar al-Assad's autocratic rule.
And UN rights chief Navi Pillay was invited to visit Damascus, a spokesman said, urging authorities to probe the killing of protesters.
"We look forward to being able to visit and independently assess the situation on the ground," Rupert Colville said.
"This barbaric behaviour is aimed at keeping the regime in place at the expense of civilians who are killed... The Security Council must convene rapidly to stop the bloodshed."
Amnesty International chief Salil Shetty said the Syrian regime was "clearly trying to shatter the will of those peacefully expressing dissent by shelling them, firing on them and locking them up.
"The Syrian government and its security forces have long felt able to operate with total impunity, and we are now seeing the result of that in the kinds of bloody acts that they have been committing on the streets of Syria in recent days," she said.
"The Security Council needs now to step up to the mark and show leadership on Syria as it did on Libya.
"A consistent policy of zero-tolerance for crimes against humanity will send a signal to all governments that impunity for crimes under international law is no longer acceptable," Shetty said.
Activist Abdullah Abazid told AFP by telephone from Daraa that "new army troops and security reinforcements have entered" the southern town on Tuesday.
Troops were firing on residents and a mosque and also besieging the home of Muslim cleric Mufti Rizk Abdulrahman Abazeid, who quit last week in protest at the crackdown.
"The bullets continue against the people, but we are resisting," he said.
On Monday, 3,000-5,000 Syrian troops backed by tanks and snipers rolled into Daraa, residents and Abazid said.
Farther north in the protest hub of Banias, thousands took to the streets on Tuesday, chanting "freedom, freedom," amid reports an assault was imminent.
"We warn the corrupt security services against attacking our city Banias" like they did in Daraa, Sheikh Anas Ayroot, a protest leader, told protesters.
One rights activist who declined to be identified suggested that forces had deployed on hills around Banias in preparation for an attack.
Security forces also deployed in the northern Damascus suburb of Douma where they set up identity checkpoints, a witness told AFP.
The Douma crackdown began on Monday and also targeted nearby Al-Maadamiyeh.
By Tuesday afternoon, Douma had become a "ghost town," one resident told AFP by telephone.
Security forces arrested three doctors from Douma's Hamdan hospital and forced patients to leave, even those in intensive care, witnesses said.
Also on Tuesday, authorities referred to a military court prominent dissident Mahmud Issa for owning a satellite phone, after he was interviewed by Al-Jazeera, said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
Issa was twice jailed in Syria for a total of 11 years.
He said activist Qassem Azzawi was arrested on Tuesday for joining a protest last week, and that 43 people had been rounded up since Monday in countrywide raids but 11 others were freed on Tuesday.
Activists said at least 25 people were killed in Daraa on Monday, and Abazid said the bodies of 22 people had been retrieved.
The Daraa operation came four days after Assad scrapped nearly five decades of draconian emergency rule and abolished the repressive state security court as he faced growing dissent and calls for reform.
The army said troops entered Daraa "in response to calls for help" from citizens to rid them of "extremist terrorist groups" behind a spate of killings and sabotage.
The United States ordered the evacuation of non-essential embassy staff from Damascus, where the first US ambassador in six years took his post just three months ago.
On Monday a spokesman for the US National Security Council said Washington was considering imposing "targeted sanctions" against Damascus.
Britain, Italy and France meanwhile denounced the "violent repression" as British Foreign Secretary William Hague said London was working with the United Nations and European Union to send a "strong signal" to Damascus.
France called for "strong measures", a foreign ministry spokeswoman said, as President Nicolas Sarkozy branded the situation "unacceptable."
Turkish Prime Minister Recep Tayyip Erdogan, who has close ties with Assad, called him on Tuesday to press for reform, an aide said.
Diplomats say Britain, France, Germany and Portugal are seeking UN condemnation of the killing of demonstrators and a call for an independent investigation.