Saudi-led coalition warplanes bombed on Saturday Houthi fighters from Yemen seeking to infiltrate Saudi Arabia, killing tens of Houthi militiamen, security sources said.

The bombing took place on the Yemeni side of the border close to the Saudi city of Najran, they said. Clashes were also seen in the northwestern Yemeni town of Haradh which borders the kingdom, Reuters reported according to witnesses.

The escalation in violence was one of the worst since peace talks began in Kuwait in April between Yemen’s government and the Houthis to end a 16-month conflict that has left more than 6,400 people dead, nearly half of them civilians, and displaced more than 2.5 million.

A truce that began in April has slowed the momentum of fighting, but violence continues almost daily.

Prospects for progress in the talks dimmed further this week when Houthi rebels and their allies in the political party of former president, Ali Abdullah Saleh, announced they had decided to form a political council to unilaterally rule the country.

Ismail Ould Cheikh Ahmed, the United Nations Yemen envoy, said the move gravely violated U.N. Security Council Resolution 2216, which calls on the Houthis “to refrain from further unilateral actions that could undermine the political transition in Yemen”.

YEMEN GOVERNMENT WITHDRAWS FROM TALKS

Yemen’s government delegation has quit talks in Kuwait with Shi’ite Houthi rebels despite resuming them earlier this month.

Officials with Yemen’s internationally recognized government of President Abed Rabbo Mansour Hadi told The Associated Press they were considering proposals to reconvene the talks, which have failed to bridge the gap with the rebels, in Oman.

Hadi’s negotiators are demanding the implementation of a U.N. Security Council resolution which stipulates the withdrawal of militias from all cities.