A pair of assassinations of anti-Israel militant leaders hours apart is threatening to set off a regional clash and upend already fragile talks aimed at ending the war in Gaza.

The deadly round of strikes, retaliation and negotiations escalated Wednesday when Hamas’ political chief, Ismail Haniyeh was killed hours after he attended the inauguration of Iran’s new president in Tehran. Israel has not claimed responsibility, but Iran claims the United States had a role, and threatened revenge against Israel.