Doug Stanglin

USA TODAY

Syrian activists say the Islamic State re-entered the historic city of Palmyra in central Syria on Saturday, pushing back the Russian-backed Syrian army that expelled the group only nine months ago in a highly publicized campaign.

The Britain-based Syrian Observatory for Human Rights reports that the Islamic State, also known as ISIL or ISIS, "entered Palmyra on Saturday and now occupies its northwest. There is also fighting with the army in the city center," Al Jazeera reports. The observatory says the militants reached the city’s Tadmor Hospital and its strategically located wheat silos.

The activist-run Palmyra Coordination Collective also reports militants seized the city’s military warehouse and its northern and western neighborhoods after taking several government positions, oil fields, and strategic hilltops in the surrounding countryside in a rapid three-day campaign, the Associated Press reports.

When the Islamic State took control of Palmyra in July 2015, it destroyed several of the city’s famed ancient Roman monuments and executed its archaeological director.

The new advances by the group contrast sharply with its defeats in territory it held in Iraq as part of the self-proclaimed caliphate that straddles two countries. Iraqi troops, aided by a U.S.-led coalition, have pushed the militants out of key cities in Iraq and retaken much of Mosul, the group's last major stronghold in Iraq.

The fight in Palmyra comes one day after U.S.-led coalition forces announced U.S. warplanes destroyed a fleet of 168 oil tanker trucks near the ancient city in the largest such strike against the terror group's finances this year.

More than 20 aircraft were used to destroy the tanker trucks. The oil in the trucks was worth about $2 million, according to a statement.

The airstrike, launched Thursday, is part of an operation to reduce revenues for the militant group. The Islamic State gets 25% of its revenue from oil, the Center for the Analysis of Terrorism, a private research group, estimates.