Syrian troops and tanks rolled into the protest flashpoint town of Daraa on Monday, drawing criticism from the United Nations.
Witnesses in southern Syria say the security forces have killed at least five people, as authorities widen their crackdown on a 5-week anti-government uprising.
The witnesses said they saw bodies in a car that had been attacked by security forces in Daraa. Video footage distributed by human rights groups shows tanks in the city along with dozens of soldiers in a show of military might.
Rights groups also reported gunfire from security forces in Douma, a suburb of the capital, Damascus.
U.N. High Commissioner for Human Rights Navi Pillay demanded that Syria curtail its military crackdown and investigate the killings of protesters.
Also on Monday, Jordanian officials reported that the border between the two countries has been sealed and Syria is preventing people from crossing. Syrian officials denied the report.
Meanwhile,The Wall Street Journal reported that the Obama administration is readying sanctions against Syria, including freezing the assets of top Syrian officials.
On Sunday, rights groups said security forces killed at least four civilians and detained many opposition activists in the coastal city of Jableh, following a pro-democracy protest against President Bashar al-Assad the previous night.
Witnesses say the killings occurred even though no protests were in progress at the time.
Sweeps against anti-government protesters have escalated in Syria despite last week's repeal of a nearly 50-year-old emergency law. The New York Times, quoting the executive director of the Syrian rights group Insan Wissam Tarif, said 217 people have disappeared since Friday.
Funerals were held for those killed in the violence Friday and Saturday. Rights groups say at least 120 people were killed in the 2-day crackdown.
The U.S.-based group Human Rights Watch Sunday called for a U.N. inquiry into the deaths and for international sanctions on the officials responsible for the killings.
President Assad signed a decree ending decades of emergency rule last week. The ruling was part of his effort to end anti-government unrest by meeting a key demand of protesters.