WASHINGTON — Hundreds of Nigerian soldiers are unaccounted for after Islamist fighters disguised in Nigerian military uniforms overran a remote base Saturday, according to two U.S. officials and an individual with knowledge of the incident.

The Nigerian army has emphatically denied that the soldiers are missing, but the U.S. officials said they remain unaccounted for. The individual with knowledge of the incident said that so far at least 100 have been found in neighboring areas or have returned to the base. The individual also said, however, that Nigerian officials do not yet know if up to 500 others were killed, captured or simply fled.

Nigerian officials did not immediately respond to requests for comment.

According to the U.S. officials, the Nigerian military had intelligence that terrorists were planning to attack the new forward operating base in northeastern Nigeria, and had called in reinforcements.

Around 7 p.m. local time, a group of Islamist fighters approached the base near the village of Jilli. They were dressed in Nigerian military uniforms and drove technical vehicles painted to look like Nigerian military trucks.

A map showing the location of Jilli, Nigeria. Bing Maps

While the Nigerian government has suggested the fighters were linked to Boko Haram, the U.S. officials said they believe they were with a different Islamist group, ISIS West Africa.

The guards allowed the fighters on the base, thinking they were the expected reinforcements. "They let them right in the front door," a U.S. official said.

The attack surprised and overwhelmed the troops, and fighting lasted for about two hours before the militants left the base.

About 730 Nigerian troops were on the base when the attack began. More than 600 were missing just after the attack, according to the U.S. officials. The Nigerian military said it had recovered dozens of bodies and found dozens wounded in nearby areas. Other soldiers apparently fled from the assault and then returned to the base, according to the individual with knowledge of the incident.

According to the U.S. officials, the attacking militants also took vehicles, arms and munitions when they left the base.

In a statement Monday, the Nigerian military denied a report in local media that hundreds of soldiers had gone missing.

"The Nigerian Army wishes to state categorically that the report is not only untrue but capable of misleading members of the general public," said Brig. Gen. Texas Chukwu, director of public relations for the army.

"The write-up [alleged] that about 100 of the troops that disappeared from the base in Jilli after the clash have reported in Geidam town but very few of them had their weapons with them.

"Contrary to the report, the army wishes to state that, although the base was attacked by suspected Boko Haram insurgents, troops reorganized and successfully repelled the attack and normalcy has since returned to the area. Also, all the troops in the base were accounted for contrary to the report."

On Wednesday, Chukwu issued a second statement saying that Nigeria's chief of army staff had visited the area in the preceding 48 hours to assess the situation. He urged the media to "be very cautious when reporting some of these developments that have national security implications." He did not provide any new information about casualties, but said that while the attack was "very unfortunate," it had been "brought under control."

The outpost in Yobe State near the border between Niger and Nigeria was established to stop the flow of terrorists, mainly Boko Haram, through the Lake Chad Basin area.

Nigeria requested help from the U.S. to recover vehicles using satellite and surveillance imagery, said the two U.S. officials, but the officials said they were not aware of that request being approved.

In a statement, a spokesperson for U.S. Africa Command (AFRICOM) said the U.S. is aware of reports about the attack, but had no information to provide. "That said, we stand ready to provide assistance if and when requested, or as appropriate."

The U.S. military conducts a train, advise, and assist mission in Nigeria, but no U.S. troops were present at the time of the attack. According to the AFRICOM spokesperson, the U.S. has about 50 Defense Department personnel, including service members, civilians and contractors, in Nigeria.

The attack came one day after an attack against a Nigerian military convoy in neighboring Borno State.

Eleven military vehicles were attacked by Boko Haram fighters. Three vehicles made it out of the firefight and 23 Nigerian troops are still missing.