BEIRUT/Washington - A US-backed alliance of Arab-Kurdish forces entered the key Islamic State-held town of Tabqa on Monday as they pursued their campaign against the IS group in northern Syria.
The Syrian Democratic Forces have set their sights on Tabqa and the adjacent dam as part of their broader offensive for the city of Raqa, the Syrian heart of the militants' self-styled "caliphate" since 2014.
Supported by US-led coalition air strikes and special forces advisers, the SDF surrounded Tabqa in early April. On Monday, they entered it for the first time, the Syrian Observatory for Human Rights monitoring group said.
"They seized control of several points in the town's south and were advancing on its western edges," said Observatory head Rami Abdel Rahman.
The US-led coalition warplanes carried out "intense" strikes in support of the offensive, he said, adding that one raid had killed seven children and four women trying to flee Tabqa.
In an online statement, the SDF said it had captured IS-held positions in west Tabqa, including a roundabout, and part of a southern district. "There are now clearing operations in the liberated positions," the SDF said.
Tabqa sits on a key supply route about 55 kilometres (34 miles) west of Raqa, and served as an important IS command base, housing the group's main prison.
According to the Syrian Economic Task Force, a Dubai-based think tank, Tabqa is home to 85,000 people including IS fighters from other areas. The assault on Tabqa began in late March when SDF forces and their US-led coalition allies were airlifted behind IS lines.
The ensuing fight has been intense, with IS dispatching suicide bombers daily to try to slow the offensive and coalition warplanes intensifying their raids. "The real battle begins now," Abdel Rahman said on Monday, adding that IS fighters had "no way" out of the town.
For months, the SDF has been advancing on Raqa, hoping to encircle it before a final attack.
The city was home to around 240,000 residents before 2011, and more than 80,000 people have fled to it from other parts of the country. Syria's war has left more than 320,000 people dead since it began with protests in 2011 that were brutally repressed by the government of President Bashar al-Assad.
Regional and international powers have since been drawn into the complex conflict, in which internationally prohibited weapons such as cluster bombs and toxic gas have been used. On April 4, a suspected chemical attack killed 88 civilians, including many children, in the northwestern rebel-held town of Khan Sheikhun.
Much of the international community blamed the Syrian regime, and three days later 59 US cruise missiles targeted the airbase from where the attack was launched.
Meanwhile, the US government put 271 Syrian chemists and other officials on its financial blacklist Monday, punishing them for their role in the deadly chemical weapons attack on a rebel-held town in early April.
In one of its largest-ever sanctions announcements, the US Treasury Department took aim at the Syrian Scientific Studies and Research Center (SSRC), which it said was responsible for developing the alleged sarin gas weapon used in the April 4 attack.
The attack left 87 dead, including many children, in the town of Khan Sheikhun, provoking outrage in the West, which accused Syrian President Bashar al-Assad of being responsible. The sanctions include a freeze on all assets in the United States belonging to 271 individuals on the blacklist, and block any American person or business from dealing with them.
According to the Nuclear Threat Initiative, a Washington-based think-tank, the SSRC is Syria's leading scientific research center, with close links to the country's military.
The center itself was already the subject of two sanctions declarations, in 2005 and 2007, due to its alleged role in developing weapons of mass destruction.
The Treasury said in a statement Monday that the SSRC is specifically behind the Syrian government's efforts to develop chemical weapons and the means to deliver them.
The 271 either have specific scientific expertise for the program or have been involved in it since 2012, it said.
"These sweeping sanctions target the scientific support center for Syrian dictator Bashar al-Assad's horrific chemical weapons attack on innocent civilian men, women, and children," Treasury Secretary Steven Mnuchin said.
"The United States is sending a strong message with this action that we will hold the entire Assad regime accountable for these blatant human rights violations in order to deter the spread of these types of barbaric chemical weapons."
The Treasury's action followed the US military's firing 59 Tomahawk cruise missiles at a Syrian airfield on April 7 to punish the government and set a warning against any further chemical weapons attacks.
"These sanctions are intended to hold the Assad regime and those who support it - directly or indirectly - accountable for the regime's blatant violations of the Chemical Weapons Convention and UN Security Council Resolution 2118," Mnuchin said.
The attack on Khan Sheikhun was also debated in the UN Security Council. But Russia, a close ally of Assad, vetoed a resolution on April 12 demanding the Syrian government cooperate with an investigation of the attack.
Assad has denied the attack, branding it a "fabrication" by the West.
The US Treasury already had imposed sanctions against 18 Syrian officials in January. The sanctions effectively aim to shut those targeted out of global financial networks, making it difficult for them to do business or even travel.
Mnuchin said the administration "will relentlessly pursue and shut down the financial networks of all individuals involved with the production of chemical weapons used to commit these atrocities."