Meanwhile, Jordan’s information minister, Taher Adwan, confirmed the report of border closings, but a Syrian channel quoted the Syrian customs director as saying the report was “baseless.”

“Syria closed its land borders with Jordan. The Syrian decision is related to the internal situation in Syria,” Mr. Adwan told the state-run Petra news agency, according to Reuters.

A senior diplomat in the Jordanian capital earlier confirmed that the two main Syrian crossings at Deraa and Nassib on the Syrian side were closed to traffic. An official told Reuters the “timing is related to what appears to be a major security operation that is taking place right now.”

The residents of Deraa have ties with tribes living just few kilometers inside the Jordanian territories. There were reports that witnesses talking to international media were using Jordanian cell phones.

Witnesses and human rights activists told Al Arabiya that the battle was becoming fiercer in Deraa.

One of them spoke to Al Arabiya about “clashes” between special security forces and members of the army in Deraa. He added that “casualties” have been reported.

Al Arabiya could not confirm these reports, as it was almost impossible to get an official statement from Damascus.

Earlier on Monday, thousands of troops backed by armor swept into the flashpoint Syrian town Deraa, killing and injuring an unknown number of people and leaving bodies lying in the streets, activists said.

Eight tanks and two armored vehicles deployed in the old quarter of the besieged city in a main street near the Omari mosque, a witness said.

Other rights activists said there were also security force raids in the towns of Douma and al-Muadamiyah near Damascus in the latest crackdown on a five-week uprising against President Bashar al-Assad’s authoritarian regime.

Rights activists told Agence-France Presse that a 3,000-strong military force swarmed into Deraa early Monday, with tanks taking up positions in the town center and snipers deploying on rooftops.

The activists said they were not able to count the casualties as the snipers made it impossible to reach them.

“Bodies are lying in the streets and we can’t recover them,” one activist said.

“People are taking cover in homes. I could see two bodies near the mosque and no one was able to go out and drag them away,” a witness told AFP.

Deraa has become an epicenter of the protest movement, which kicked off in the town more than a month ago after authorities arrested a group of teenagers who scrawled anti-regime graffiti on a wall. Since then, more than 300 people have been killed across the country of 23 million people as the anti-government demonstrations have swelled.

Activists on social media posted footage of what they said were troops firing early Monday throughout Deraa, The Associated Press reported. The sound of heavy gunfire punctuated the footage, as well as the labored, frightened breathing of the activist filming the footage. The activist repeated the date and the location and said: “The army forces are entering Deraa. They are shelling the city of Deraa.”

The video could not be independently verified and all witnesses spoke on condition of anonymity for fear of reprisals, according to AP. Syria has banned nearly all foreign media and restricted access to trouble spots since the uprising began, making it nearly impossible to get independent assessments of the situation on the ground.

But the rising level of violence in Syria—more than 120 people have been killed since Friday—brought calls from the watchdog group Human Rights Watch for a UN inquiry.

The assaults by Mr. Assad’s forces recalled a similar operation conducted by his late father, President Hafez al-Assad, against the Muslim Brotherhood in Hama in 1982. The numbers of deaths in that operation were estimated to be 20,000.

Syrian security forces have detained dozens of opposition activists since Saturday as authorities turned to pinpoint raids to quell the revolt.

The raids were concentrated around the capital Damascus and the central city of Homs, a hotbed of demonstrations against President Assad's authoritarian rule, said Ammar Qurabi, head of the National Organization for Human Rights in Syria.

“These people are not being arrested in a legal way. They are being kidnapped,” Mr. Qurabi told AP, claiming the plainclothes security agents did not have formal arrest warrants.

The Syrian Observatory for Human Rights, meanwhile, said that the Syrian security forces have shot dead at least 13 civilians since they swept into the coastal town of Jabla on Sunday, according to AFP.

The security forces and gunmen loyal to President Bashar al-Assad deployed in the old Sunni quarter of Jabla on Sunday after a pro-democracy protest in the town the previous night, rights campaigners in contact with Jabla said.

Mr. Assad, 46, has blamed most of the unrest on a “foreign conspiracy” and armed thugs trying to sow sectarian strife. The state-run news agency SANA said 286 police officers have been wounded since the uprising began. It did not give further details.

The crackdown by the security forces comes despite President Assad signing on Thursday decrees ending a draconian state of emergency, imposed by the Baath Party when it seized power in 1963, to placate more than a month of pro-democracy protests.

He also abolished the state security court that has tried scores of regime opponents outside the normal judicial system and issued a decree allowing citizens to hold peaceful demonstrations.

The Syrian Revolution 2011 group, a driving force behind the protests, indicated it would keep up the pressure. “We are going out (on the streets) today, tomorrow and the day after,” said a statement on its Facebook page, according to AFP.

The US government is preparing sanctions against senior officials in the entourage of President Assad who are overseeing a violent crackdown against protesters in the country, The Wall Street Journal reported late Sunday.

Citing unnamed officials, the newspaper said the administration of Barack Obama was drafting an executive order empowering the US president to freeze the assets of these officials and ban them from any business dealings in the United States.

Unilateral sanctions by Washington on Syrian officials wouldn’t have much direct impact on Mr. Assad's inner circle, as most regime members have few holdings in the United States, the Journal said.

But countries in Europe, where the Assads are believed to have more substantial assets, will be pressured to follow Washington’s lead, according to the report