U.S. military leaders and elected officials are grappling with a worsening diplomatic and security crisis in the Middle East.

The fighting between Israel and Iran intensified on Monday night, as both nations kept up their attacks, killing civilians and raising concern among world leaders that the biggest battle between old enemies could lead to a broad regional conflict.

Israel PM Netanyahu warned the offensive could last weeks and urged Iranians to oppose their rulers, fueling fears of a broader regional war.

The latest US-Iran talks on Tehran’s rapidly advancing nuclear programme will not take place on Sunday, mediator Oman said on Saturday, as Israel and Iran traded blows a day after Israel’s blistering attack on Iranian nuclear and military sites.

The retaliatory strikes came on Saturday, a day after Israel killed top Iranian military leaders and scientists and destroyed an aboveground nuclear enrichment plant near Natanz.

Iran’s supreme leader has criticized an initial proposal from the United States in negotiations over Tehran’s rapidly advancing nuclear program.

The escalating conflict between Israel and Iran is likely to be high on the agenda as world leaders gather in Canada on Monday.

The two sides were scheduled to meet for a sixth round of negotiations on Sunday. Oman, which was set to host the talks, said on Saturday that this round had been canceled.

Iranian officials on Saturday signaled they could walk back their decision to not attend upcoming nuclear talks with the U.S., but said further negotiations were likely “meaningless”

Amid the escalating violence, US President Donald Trump said he and Russian President Vladimir Putin are in agreement that the hostilities must end immediately.