The United States is concerned about indications that Nigeria’s Boko Haram jihadists are sending fighters to join Islamic State in Libya in increased cooperation between the two groups, a senior U.S. official said on Friday.

Boko Haram, which has been waging a seven-year insurgency in northern Nigeria, last year pledged loyalty to Islamic State, which has seized parts of Libya, Syria and Iraq. But little has emerged about the extent of a cooperation.

“What we’ve seen are reports of more cooperation between them,” Deputy Secretary of State Antony Blinken told reporters on a visit to Nigeria.

“We’ve seen that Boko Haram’s ability to communicate has become more effective. They seem to have benefited from assistance from Daesh,” he said, using a derogatory Arab name for Islamic State.

“There are reports of material and logistical assistance,” Blinken added.

He said there were “reports” that Boko Haram fighters were going to Libya, where Islamic State has a large presence in Sirte, exploiting a security vacuum as competing governments and armed groups fight for power.

“So these are all elements that suggests that there are more contacts and more cooperation and this is again something that we are looking at very carefully because we want to cut it off and we want to prevent it from going any further.”

On Thursday, Boko Haram claimed  it was behind a suicide bombing in the Nigerian city of Maiduguri, killing five people, according to a statement published in Arabic in the name of Islamic State West Africa.