Jordanian warplanes hit and destroyed several vehicles trying to cross the border from Syria, a government spokesman said on Wednesday, underlining Amman's concern about incursions from areas controlled by Syrian rebels.

"There was an attempt to infiltrate across the border from Syria by a number of vehicles," said spokesman Mohammad al-Momani, also a cabinet minister, told Reuters.

A Jordanian security source said the targets were Syrian rebels in civilian cars mounted with machine guns.

It was unclear whether rebels who have dominated swathes of territory along Syria's southern border with Jordan might have been seeking refuge inside Jordan from fighting with President Bashar Assad's forces that has raged in the region.

Momani said the kingdom was increasingly worried about infiltration from Syria. "We are worried about cases of infiltration ... and reports that talk about armed groups that are close to the border and the absence of security there."

An army statement released said the attack happened Wednesday at 10:30 A.M. (0730 GMT). The statement says the camouflaged vehicles were driving in a rugged area near the border and ignored demands to stop from security forces.

The statement said Jordanian warplanes fired warning shots at the vehicles. It added that the vehicles did not stop and were then destroyed in airstrikes. The statement did not say how many vehicles were destroyed, nor did it offer casualty figures.

Jordanian state television reported the airstrikes without elaborating.

Amman has tightened already firm controls along the 370-kilometer (230 mile) border to try to prevent Jordanian Islamist militants who have joined the rebels from crossing back into Jordan. They are seen as a domestic security threat.

Jordan's armed forces routinely arrest smugglers trying to cross its desert border with Syria, but such airstrikes are rare.

"No vehicles belonging to the Syrian Army moved towards the Jordanian border and what was targeted by the Jordanian Air Force does not belong to the Syrian Army," Syrian state news agency SANA said, quoting an unnamed military source.

Four killed in Syrian airstrike on rebel-held town

Syrian military airstrikes killed at least four people early Wednesday in a rebel-held town along the Lebanese border, activists said, as pro-government forces intensify their campaign against some of the last rebel strongholds on a valuable supply line.

The shelling hit rebels on the edge of the town of Zabadani and wounded 10 people, said Rami Abdurrahman of the Britain-based Syrian Observatory for Human Rights. He and a Damascus-based activist Ammar al-Hassan said the strike came during intensified shelling of the town.

Zabadani is in a part of Syria protruding into the Bekaa Valley in eastern Lebanon. The town and nearby Madaya are on the Qalamoun frontier with Lebanon, areas that once served as opposition supply routes to nearby rural Damascus.

Syrian forces, bolstered by fighters from the Lebanese Shiite militant group Hezbollah, systematically took back most other rebel-held towns along the mountainous frontier in a campaign that began in November.

On Wednesday, Syrian forces loyal to President Bashar Assad took the town of Housh Arab, the state-run SANA news agency reported. It fell after pro-Assad forces took the nearby town of Arsal al-Ward on Tuesday.

Rebels still hold the town of Talfita in Qalamoun, but it is now surrounded by Assad-held territory.

Al-Hassan said those in Zabadani helped smuggle wounded fighters into the nearby Sunni Lebanese town of Majdal Anjar, some 15 kilometers (9 miles) away, and allowed rebels to smuggle supplies through the town.

"From the days of the peaceful protests of the revolution, it was a chief smuggling place and it remains that way," al-Hassan said.

Still, al-Hassan and another activist Akram al-Shami said while they expected Assad forces to retake Zabadani, government-backed forces face difficult conditions.

The town sits on a hill and is isolated from other parts of the Qalamoun, meaning it will be difficult amass ground troops and Syrian forces will have to rely mostly on air power, they said.

"They have to cross a lot of valleys and mountains," al-Shami said.