International mediators were heading to the Middle East for a high-stakes round of negotiations scheduled for Thursday as they raced to lock down an elusive cease-fire in the Gaza Strip that could defuse tensions ahead of an anticipated attack on Israel by Iran and Hezbollah.

The cease-fire talks, which are set to take place in Doha, Qatar, or Cairo, were expected to include top intelligence officials from Egypt, Israel and the United States, as well as the Qatari prime minister.

But as of Tuesday, Hamas representatives were not planning to take part. Ahmad Abdul-Hadi, a Hamas representative in Lebanon, said in an interview that doing so would mean going “backward to square one,” and accused Prime Minister Benjamin Netanyahu of Israel of dragging out the negotiations.

“Netanyahu is not interested in reaching an agreement that ends the aggression completely, but rather he is deceiving and evading and wants to prolong the war, and even expand it at the regional level,” Mr. Abdul-Hadi said.

Hamas’s decision did not appear to bode well for a breakthrough on Thursday, but it did not mean the group had completely left the bargaining table.

Hamas leaders have not met directly with Israeli officials throughout the war, relying on Qatar and Egypt to act as intermediaries. And many of Hamas’s most senior political leaders are based in Qatar, a short drive from the offices of Qatari mediators in Doha.