(Reuters) - U.S.-led forces launched air strikes on Islamic State-controlled territory in northern and eastern Syria overnight and the Syrian army continued bombing areas in the west, a group monitoring the war said on Monday.

The United States has been carrying out strikes in Iraq against the militant group since last month and in Syria since last week with the help of Arab allies. It aims to damage and destroy the bases and forces of the al Qaeda offshoot which has captured large areas of both countries.

The raids hit Islamic State in the northern Syrian town of Manbij, said the Britain-based Syrian Observatory for Human Rights, which gathers information from sources in Syria.

Manbij sits between Aleppo city in the west and the town of Kobani on the northern border with Turkey, which Islamic State has been trying to capture from Kurdish forces, forcing tens of thousands of Syrian Kurds to flee over the frontier.

Syria's army also carried out air raids in Aleppo province overnight, targeting areas east of Aleppo city with barrel bombs and other projectiles, the Observatory said. The army also carried out air strikes in Hama in western Syria.

Forces loyal to President Bashar al-Assad have been battling Islamist fighters around Aleppo, which is held by a number of groups in Syria's war

In eastern Syria, U.S.-led forces bombed a gas plant controlled by the Islamic State outside Deir al-Zor city, wounding several of the militant group's fighters, the Observatory said.

The United States has said it wants strikes to target oil facilities held by Islamic State to try to stem a source of revenues for the group.

The raid hit Kuniko gas plant, which feeds a power station in Homs that provides several provinces with electricity and powers oil fields generators, the Observatory said.

U.S.-led warplanes also hit areas of Hasaka city in the north east and the outskirts of Raqqa city in the north, which is Islamic State's stronghold.