BEIRUT/ASTANA: Dozens of civilians have died in two days of intense U.S.-led strikes on Raqqa, an activist group said Tuesday, as fighting to retake the Syrian city from militants nears its densely populated center.

The coalition acknowledges it has pounded the city and surrounding area with more than 250 airstrikes over the past week alone, in support of the Syrian Democratic Forces alliance battling Daesh (ISIS).

The SDF has so far captured just under 60 percent of Raqqa, monitors say, leaving Daesh with about 10 square kilometers in the heart of the city.

But as clashes approach central Raqqa, monitors and activists have reported scores killed in intensifying coalition bombardment of the city.

Monday, U.S.-led airstrikes killed at least 42 civilians in several neighborhoods in Raqqa under Daesh control, according to the Britain-based Syrian Observatory for Human Rights.

Nineteen children and 12 women were among the dead, the group said.

The Observatory says 167 civilians have been killed in coalition strikes since Aug. 14, including 27 Sunday. “The tolls are high because the airstrikes are hitting neighborhoods in the city center that are densely packed with civilians,” Observatory director Rami Abdel-Rahman said.

“There are buildings full of civilians who are trying to get away from the front lines.”

The coalition says avoiding civilian casualties is its “highest priority.”

Speaking during a visit to Baghdad, U.S. Defense Secretary Jim Mattis insisted Daesh tactics were to blame for placing civilians in danger.

“We are the good guys and the innocent people on the battlefield know the difference,” he said.

But the coalition acknowledges a major uptick in its strikes on Raqqa, with more aircraft available since a U.S.-backed operation successfully pushed Daesh from Mosul in neighboring Iraq last month.

Coalition spokesman Col. Ryan Dillon said more than 250 strikes had hit Raqqa and its surroundings in the last week, and an AFP count of the coalition’s own reporting put the figure at over 300 strikes.

“It’s probably logical to assume there has been some increase in civilian casualties. But I would ask someone to show me hard information,” said Lt. Gen. Stephen Townsend, who commands the coalition.

Dillon told AFP that the latest allegations of civilian deaths would be taken seriously and investigated.

The coalition, which operates in both countries, earlier this month acknowledged the deaths of 624 civilians in its strikes in Syria and Iraq since 2014, but rights groups say the toll is much higher.

The SDF’s Arab and Kurdish fighters broke into Raqqa in early June after spending months chipping away at Daesh-held territory in the surrounding province.

Activist collective Raqqa is Being Slaughtered Silently (RBSS) also reported heavy raids in recent days.

“Unfortunately, civilians have no way to protect themselves,” RBSS’ Husaam Essa said.

“All they can do is try to hide in whatever shelter they can and avoid going out into the street as much as possible,” he told AFP.

More than 330,000 people have been killed since Syria’s conflict erupted in March 2011 with protests against President Bashar Assad.

Tuesday a meeting between Syrian opposition groups in Riyadh has ended in stalemate, a member said, with the fate of Assad still an obstacle in forming a unified front for peace talks.

The Saudi-backed opposition High Negotiations Committee (HNC) began discussions Monday with delegations from two other moderate camps, the so-called Cairo and Moscow groupings, in a bid to reach consensus on a joint negotiating strategy.

After hosting seven rounds of largely unsuccessful talks, U.N. Syria envoy Staffan de Mistura had sought to unify the opposition for what he hopes will be a substantive round of negotiations in October.

“The representatives of the Moscow grouping refused to recognize any text that referred to the Syrian people’s demand for the departure of Bashar Assad,” said Ahmed Ramadan of the National Coalition, a leading component of the HNC.

“There was an important level of understanding between HNC and the Cairo grouping, but the stalemate with Moscow group delegates hampered efforts to bring representatives ... into a single negotiating delegation.” There was no immediate comment from the so-called Moscow group.

Assad’s fate has long been a key sticking point, with the HNC insisting on his ouster but the other two camps adopting a softer stance.

De Mistura said last week that he hopes for “real” peace talks between the government and a still-to-be-formed unified Syrian opposition in October.

Alongside the negotiations in Geneva have been peace talks in Astana, where a new round was slated to be held later this month.

But Tuesday Kazakhstan said that the next talks in its capital Astana may be held in mid-September.

Kazakhstan’s Foreign Ministry wrote on Facebook that the timing of the talks would be set at a meeting this month between experts from Russia, Turkey and Iran and “provisionally, we could be talking about mid-September.”