LAGOS, Nigeria — The effort to rescue the 276 schoolgirls believed to be captives of Islamic extremists in northeastern Nigeria was boosted Friday when British security experts joined the Nigerian and American forces trying to rescue the missing students.

Britain said its aim was not only to help with the current crisis but to defeat Boko Haram.

“The team will be considering not just the recent incidents but also longer-term counter-terrorism solutions to prevent such attacks in the future and defeat Boko Haram,” the Foreign & Commonwealth Office said in a statement Friday.

The American team was joined by six additional military officers and more are expected soon, said Pentagon spokesman Navy Rear Adm. John Kirby. The U.S. officers will do a “gap analysis,” an assessment to identify what the Nigerian military needs that the U.S. could provide in the search for the girls, he said.

In New York, the U.N. Security Council hinted at sanctions against Boko Haram. In a strongly worded statement, the council condemned a May 5 attack that killed and injured hundreds and demanded the immediate release of the kidnapped girls.

The weakness of the Nigerian armed forces was highlighted Friday in a report issued by Amnesty International which said the military did not respond to warnings that Boko Haram rebels were about to attack Chibok, the town where the young women were abducted from their school.

Boko Haram has staged many attacks in northeastern Nigeria over the years, a campaign of bombings and massacres that has intensified in recent times. Since May 2013 there has been a state of emergency in three northeastern Nigerian states wracked by Boko Haram violence.