WASHINGTON - More mosques in the United States received threatening letters warning Muslims to leave the country or face extermination, according to media reports.

Masjid Al-Kareem in Providence, Rhode Island, is the latest Muslim place of worship to receive a hate-filled letter referring to Muslims as "vile and filthy people." Police in Providence said they would increase patrols around the mosque after the threatening letter was received.

Faissal Elansari of the Islamic Center of Rhode Island called the letter “really alarming,” WPRI-TV reported. “You feel that wave of hate is really at your doorstep… You’re really concerned about your community members, your kids, your families,” Elansari told the local television station.

A private Islamic school in Indianapolis, Indiana, that is affiliated with an area mosque also received a hate-filled letter on Tuesday.

The Islamic Society of North America, which is based in Plainfield, Indiana, says the letter was sent to the MTI School of Knowledge, which is connected to the Masjid Al-Fajr Islamic Center of Greater Indianapolis.

Both letters were identical and said US President-elect Donald Trump would do to "Muslims what Hitler did to the Jews."

The letters were postmarked from Santa Clarita, California, on November 21, the reports said.

Hate letters have been received at six mosques in the state of California, including in Los Angeles, Fresno and San Jose. They have also tuned up in the states of Georgia, Ohio, Michigan, Colorado and Indiana. All the letters appear to be photocopies of the same handwritten note.

The Council on American-Islamic Relations, the largest US Muslim advocacy group, has asked the Federal Bureau of Investigation (FBI) to investigate. Civil rights groups have warned of a growing number of attacks against minorities, including Muslims, since Donald Trump was elected to the White House on November 8.

The cabinet appointments of Trump have signaled his intention to deliver on his hard-line campaign promises and have deepened concerns among Muslim Americans about an anti-Islamic White House.

Trump won the US presidency despite extreme unpopularity among minorities, underscoring deep national divisions that have fueled incidents of racial and political confrontation across the country.