Washington: The Obama administration is assuring governors that refugees who come to the United States in its resettlement program undergo a "rigorous security vetting process," particularly if they are fleeing from Syria.

"In short, the security vetting for this population, the most vulnerable of individuals, is extraordinarily thorough and comprehensive," Secretary of State John Kerry and Homeland Security Secretary Jeh Johnson write in letters sent to all state and territorial governors and to the mayor of Washington, DC.

A copy of the letter, dated Friday, was obtained by The Associated Press.

After the attacks in Paris and the Islamic State group's claim of responsibility, several governors vowed to block efforts to resettle Syrian refugees in the US for fear their ranks would be infiltrated by militants planning a domestic attack.

In the House of Representatives, lawmakers voted overwhelmingly to erect high hurdles for Syrian and Iraqi refugees.

The administration is countering that the vetting process is thorough and can take nearly three years. President Barack Obama has said the US will remain a welcoming place for refugees from around the world.

In their letters to governors, Kerry and Johnson say the vetting process is multi-layered and intensive and involves multiple law enforcement, national security and intelligence agencies.

They say the process includes even more precautions for Syrian refugees.

Noting that more than 4 million people have fled Syria, the American officials say some of the closest US allies and other nations are pledging to take in Syrian refugees.

They say the plan to bring at least 10,000 to the US this fiscal year "represents a modest commitment by our government." Most of the refugees, they add, are families, children and victims of torture.

A refugee applicant cannot be approved for travel and admission to the US until all required security checks have been completed and cleared, according to the letter. The vetting process includes:

An interview with the UN high commissioner for refugees to determine if the applicant meets the definition of refugee and to see if any "red flags" would render the applicant ineligible.