BEIRUT (AP) — A convoy of Islamic State militants and their relatives being evacuated from the border with Lebanon has crossed into an extremist stronghold in eastern Syria, a Syrian monitoring group said Thursday, ending a standoff with the U.S.-led coalition.

The evacuation, negotiated by Lebanon's Hezbollah group, removed the militants from the Syria-Lebanon border but angered Iraq and the U.S., which said they should have been killed on the battlefield.

The deal reached at the end of August allowed hundreds of militants and their families to relocate to Boukamal, an IS-held Syrian town near the Iraqi border, in exchange for IS-held prisoners and the remains of Lebanese soldiers captured in 2014. One surviving Hezbollah fighter was returned to Lebanon Thursday.

The U.S-led coalition struck the road the convoy was traveling on, leaving it stranded in the desert for about two weeks, though some vehicles were able to slip into militant-held territory. The U.S. said it did not strike the convoy itself because of the presence of civilians.

Last week, the U.S-led coalition said it ended surveillance of the convoy after a Russian request, as Syrian troops advanced against IS in the eastern Deir el-Zour province. Russia is one of the Syrian government's main backers, providing it with crucial air and ground support that has turned the tide of the war in the last two years.

The head of the Britain-based Syrian Observatory for Human Rights said buses and vehicles carrying about 400 militants and civilians crossed into Deir el-Zour province Wednesday.

Syrian government forces broke a three-year IS siege on parts of the provincial capital, also known as Deir el-Zour, last week, and are now battling the extremists inside the city. The militants control less than half the city and are encircled on three sides with their backs to the Euphrates River. However, they still control rural areas outside the city and the border with Iraq.

U.S.-backed Syrian forces are meanwhile advancing in the surrounding province from the east and north, on the other side of the river, setting up a race to the border with Iraq.

Bassem Aziz, a spokesman for the U.S-backed and Kurdish-led Syrian Democratic Forces, said they have taken control of an industrial area on the eastern bank of the river, a few miles from the government troops. Aziz said they are about 6 kilometers (4 miles) away from the city's eastern entrance.

The proximity of the two forces raises the specter of confrontation, as both sides vie for the border with Iraq and the oil and resources-rich province.