By BASSEM MROUE, Associated Press

BEIRUT (AP) — Syrian rebels aided by U.S.-led airstrikes launched an offensive against an Islamic State stronghold near the Iraqi border on Tuesday, hoping to sever one of the extremists' main transit links between the two countries, a rebel spokesman said.

The spokesman for the New Syrian Army group and a Syrian opposition activist said the operation will be coordinated with Iraqi tribesmen and Iraqi forces battling IS on the other side of the border. The extremist group has suffered a string of setbacks in both countries in recent weeks.

"The time has come to punish all those who were unjust to children and women, those who executed or tortured men and women," the New Syrian Army said in a statement. It called on residents of the Syrian border town of Boukamal to stay away from IS positions during the operation.

Rebel spokesman Mozahem al-Saloum said warplanes from the U.S.-led coalition launched several airstrikes on IS positions in the area Tuesday afternoon ahead of the ground offensive.

"Our aim is to cut Syria from Iraq" by capturing Boukamal and surrounding areas, al-Saloum said by telephone.

The Britain-based Syrian Observatory for Human Rights said the New Syrian Army fighters are close to Boukamal, where IS has been on high alert and shops are closed amid fears of violence. The Observatory's chief Rami Abdurrahman said the New Syrian Army has several hundred men and recently joined forces with another rebel group known as the Eastern Lions.

Omar Abu Leila, a Europe-based activist from the Deir el-Zour province, where the fighting is taking place, said the New Syrian Army and IS clashed near Boukamal, and that some IS commanders are fleeing the town amid airstrikes by the U.S.-led coalition.

IS seized and opened much of the border when it swept into Iraq from Syria in 2014, seizing large amounts of territory and declaring an Islamic caliphate. It has since shuttled fighters and weapons between the two countries.

Two weeks ago, Washington accused Russian aircraft of bombing the New Syrian Army near the Tanf border crossing with Iraq. Russia has been carrying out airstrikes in support of Syrian President Bashar Assad's forces since September.

In northern Syria, meanwhile, government forces captured farms near the city of Aleppo, tightening the noose around rebel-held parts of the city.

The Observatory, which relies on a network of activists across Syria, said government forces now control half the Mallah farms on the northeastern edge of Aleppo, Syria's largest city and former commercial center.

The Beirut-based Al-Mayadeen TV said that with the capture of large parts of Mallah farms, troops now can fire upon and effectively close the Castello road, the last remaining link between rebel-held parts of Aleppo and the rest of the country.

State media aired footage from the area, showing Syrian army tanks and artillery pounding Mallah farms. Aleppo has seen some of Syria's worst fighting this year.