BEIRUT, Sept 26 (Reuters) - More than 200 fighters have joined Islamic State in Syria's northern Aleppo province since U.S. President Barack Obama said the United States would strike the militant group in Syria, a monitoring group said on Friday.

U.S.-led forces launched air strikes on Syria on Tuesday, concentrating raids on the north and east to loosen Islamic State's grip on Syria's oil and its presence along the border with Iraq.

At least 162 people joined the radical al Qaeda offshoot in northeast and eastern Aleppo in the week after Obama's speech on Sept. 10, said the Britain-based Syrian Observatory for Human Rights, which gathers information on the conflict.

Islamic State has put particular pressure on rival insurgent groups in this part of Aleppo.

An additional 73 men had joined the group on Sept. 23 and 24 in the northeast Aleppo countryside since the start of the strikes, the Observatory said, bringing the total number since Sept. 10 to at least 235.

"This means these people are not scared. Even if there are air strikes, they still join," said Rami Abdelrahman, who runs the Observatory.

He said the pace of recruitment in September was higher than average but below the surge in July, after Islamic State declared a caliphate in the territory it controlled in Syria and Iraq and called on Muslims to join a holy war.

Most of the new men came from al Qaeda's Syrian wing, the Nusra Front, and were mostly Syrian, the Observatory said. The men who joined in the week after Obama's speech included 15 nationalities, it added.

Islamic State recruited at a record pace in July, according to the Observatory, which counted at least 6,300 new members that month, a big expansion from earlier estimates suggesting the group numbered around 15,000. Around 1,000 of the new fighters were foreign, and the rest Syrian.

Before air strikes this week, Islamic State was believed to hold roughly a third of Syria and large areas of Iraq, where U.S.-led forces have also carried out strikes.

In Syria, Islamic State has seized territory from rival Islamist groups in a belt of territory north of Aleppo, threatening rebel supply lines into the city. (Reporting by Sylvia Westall; editing by Andrew Roche)