WASHINGTON—President Barack Obama toughened his call for Israel to loosen its blockade of goods headed to Gaza, in a meeting with Palestinian President Mahmoud Abbas aimed at keeping the beleaguered Middle East peace process on track.

Mr. Obama called the situation in Gaza unsustainable and said he and Mr. Abbas had "very extensive" conversations about how to promote "a better approach" in the wake of the May 31 raid on a Turkish aid flotilla that had sought to break Israel's blockade of the territory.

Nine men were killed by Israeli soldiers in the incident.

The U.S., joining in a United Nations resolution, condemned the action and called for a credible investigation. In the aftermath, the international community has ramped up pressure on Israel to ease the blockade of the Mediterranean coastal tract.

The episode has left Mr. Abbas politically weakened by making it appear to ordinary Palestinians that his strategy of negotiating with Israel has once again proved fruitless.