LONDON — Violent extremists seeking to overthrow Syrian President Bashar Assad may instead have hurt negotiations to replace him, frustrating Western diplomats who continue to push for his ouster as a necessary part of a peace agreement in the Mideast nation’s bloody civil war.

Bolstered by infighting among Syrian opposition groups — including some linked to al-Qaida that have jeopardized foreign aid — U.S. officials say Assad has a stronger grasp on power now than he did just months ago, when the U.S. and Russia called for a new round of talks to settle the 2 1/2-year war that has killed more than 100,000 people.