WASHINGTON (Reuters) - The U.S. Homeland Security chief on Thursday presented evidence countering statements from conservative lawmakers that members of the Islamic State, which is the target of a U.S.-led air campaign in Iraq and Syria, had been caught entering America.

Homeland Security Secretary Jeh Johnson said four members of a Kurdish group that opposes Islamic State were arrested last month trying to enter the United States illegally from Mexico.

He said investigators had determined the four were members of the Kurdistan Workers' Party, known as the PKK.

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

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

Several conservative politicians and media outlets have claimed in recent days that Islamic State militants had infiltrated the United States from Mexico, which Johnson and other administration officials have 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)