A series of airstrikes near rebel strongholds outside Damascus killed at least 23 civilians Sunday as government forces tightened their grip on the capital, local officials said.

The Ghouta Media Center and the Syrian Observatory for Human Rights, which support the rebel cause, put the death toll at 23. The Syrian Civil Defense authorities — the White Helmets — said at least 16 had died in the town of Misraba alone.

"#Civil_Defense teams are working to contain the disaster after 16 civilians, including 3 children and 1 woman, were killed in an air strike targeting densely populated civilian areas in #Mesraba town in #Eastern_Goutah," the White Helmets tweeted.

The White Helmets are volunteer responders mostly called in after government air attacks, although they have aided both sides with their rescue efforts.

The airstrikes in the eastern Ghouta region came just days after conclusion of a Syrian opposition meeting in Riyadh designed to unite foes of Syrian President Bashar Assad's government ahead of U.N.-led talks Tuesday in Geneva. The opposition has dropped a long-held demand for concessions ahead of talks.

European Union foreign policy chief Federica Mogherini expressed "strong hope" that a path to peace, a new constitution and free elections could be negotiated soon.

But she acknowledged that the humanitarian crisis continued to deteriorate.

"Attacks on civilians, as seen recently in Atareb and Eastern Ghouta, have to stop," she said. "Obstacles to humanitarian access across Syria have to be removed. We count on the active support of the cease-fire guarantors Russia, Iran, and Turkey to facilitate access."

The United Nations on Saturday called for greater action to address gender-based violence and its prevention in Syria, saying it "continues to undermine the health, dignity, security and autonomy of its victims."

Last week, the U.N. issued a report stating that after almost seven years of civil war, the "complexity of needs across Syria remain overwhelming." More than 13 million women, children and men require humanitarian assistance, and 5.6 million people are in "acute need," the U.N. said.