BEIRUT (AP) — Syrian activists say at least 17 civilians, including children, have been killed in an airstrike on an underground shelter in a rebel-held area in the country's southwest.

The Britain-based Observatory for Human Rights described Thursday's airstrike in al-Musayfrah in eastern Daraa as the worst since a government offensive there began on June 19.

The Observatory says the airstrike was part of a barrage of missiles that hit the area as government troops make a push to gain territory and head toward southern Daraa and a strategic border crossing with Jordan.

Activist with the opposition-operated Horan Free Media who goes by the name Abu Mahmoud Hournai said the rescuers are still pulling bodies from the underground shelter. He put the death toll at 20, saying they included women and children.