Four anti-Islamic State Kurds arrested entering U.S. from Mexico

Johnson's announcement followed comments by conservative politicians that fighters from the Islamic State, which is the target of a U.S.-led air offensive in Syria and Iraq, had crossed the southern border.

He said investigators had determined the four people arrested in September as they tried to enter the United States illegally were members of the Kurdistan Workers' Party, known as the PKK.

The PKK is a Marxist separatist group in Turkey designated by the U.S. government as a "foreign terrorist organisation." But in the conflict against Islamic State it has allied itself with other Kurdish groups fighting the Islamist militants.

Even though the PKK was "actually fighting against ISIL and defended Kurdish territory in Iraq," the four would be deported by U.S. authorities, Johnson said in a speech in Washington.

Several conservative politicians and media outlets have claimed recently that Islamic State militants had infiltrated the United States from Mexico, which Johnson has denied.

On Tuesday, Representative Duncan Hunter, a California Republican, told Fox News that "at least 10 ISIS fighters have been caught coming across the Mexican border in Texas."

A second Republican, Representative Tom Cotton, a Senate candidate in Arkansas, said last month that the Islamic State was collaborating with Mexican drug cartels.

U.S. intelligence and national security officials said that U.S. agencies had no evidence that anyone connected with the Islamic State or other jihadist groups fighting in Iraq or Syria have tried to cross into the United States from Mexico.

The officials also said they were unaware of any threat or plots by the Islamic State or related groups coming from Mexico.

(Reporting by Mark Hosenball and Julia Edwards in Washington; editing by John Whitesides, Bernard Orr)