A U.S. visa program designed to temporarily admit religious workers from other countries may be letting jihadists into the country, security experts and religious leaders warn.

The R visa program is for non-immigrant clerics and religious workers and allows successful applicants to stay in the U.S. for up to five years. They are then allowed to apply for a permanent residency under their R-1 status.

But some critics say the visa raises red flags and has long been abused by leaders with extreme views.

"People have come in and tried to come in with this visa to preach their hardline and dangerous views, and then encourage [the] vulnerable to travel back with them where they are further brainwashed and can potentially be used to harm the USA," Adnan Khan, former president of the Council of Pakistan American Affairs, told Fox News. "The solution isn't banning innocent Muslims and migration, but looking at visas like this one which have raised red flags and caused trouble in the past."

Khan also said several letters have been written to federal agencies over the past four years concerning the R program, but they have failed to get a response.

"We want dialogue with Homeland Security and all the agencies on this," he continued. "We want to be sure no one can come here and spread hate."

The R visa program, which also extends temporary residency to spouses and unmarried children under the age of 21, worries military experts.

Col. James Williamson, who founded a group that advocates for U.S. special forces, said the program, which was created in 1990, must be tightened because of the extent to which it makes the U.S. vulnerable to jihadi attacks.

"The administration should at least temporarily suspend this dangerous loophole in our immigration process," he said. "But they should brace for backlash. In spite of applying equally to all religions, it'll be maligned and portrayed as another form of a Muslim ban."

While some have raised varying concerns over the R program, others assert that its pros outweigh its cons: Allowing foreigners to pursue their pious calling, gain extensive knowledge in the U.S., impart their existing knowledge to Americans and in turn give Americans the chance to pursue religious endeavors abroad.

"The R visa program facilitates the free exercise of religion by Americans. Most R-1 visa holders are coming to the U.S. to temporarily preach in an established U.S. church, synagogue, mosque, which is supported by American citizens," James Wolf of the California-based immigration law firm Visawolf said. "Usually their coming to the U.S. is part of an informal exchange of religious workers that occurs internationally.”

Unlike other immigrant visas, which are doled out in limited numbers, there are no quotas for the R-1 visa. Statistics regarding countries or religions that have received the visa are not documented.

Between 2012 and 2016, the U.S. issued 23,029 R-1 visas – averaging 4,605 per year – as well as 7,637 R-2 visas for spouses and children, or about 1,528 annually.

Red flags about the program have been raised for years. In 2004, seven top officials of the Holy Land Foundation – then the largest Muslim charity in the country – were indicted for providing material and upward of $12 million in financial support to the Palestinian terrorist group Hamas. The indictment stated that several of the religious leaders established the now-defunct charity by submitting false R visa applications on behalf of more than 200 immigrants from the Middle East.

Three years earlier, Muhammad Khalil, who ran a mosque in the basement of a Brooklyn store, was convicted in New York for filing over 200 falsified applications for fake jobs at his house of worship.

In 2007, after a lengthy investigation, Department of Homeland Security officials called the R visa program's fraud rate "excessively high." The DHS investigation concluded that more than 30 percent of applicants had submitted phony information or were unqualified for their positions. In some cases, their employers or places of worship did not exist, a problem that has since been somewhat rectified by official inspections and site visits.

The program has also been criticized for other reasons. DHS officials came under fire by some human rights groups and activists for allegedly targeting imams on the visa under the guise of combating terrorism.

One of those critics is William Stock, a partner at the Philadelphia-based Klasko Immigration law firm, who said that the fraud concerns that were raised resulted in heavy new regulations and procedures in 2011.

"Now, any religious worker petition requires extensive documentation of the sponsoring organization, its financial condition and membership. As well, every organization receives a 'site visit' from a USCIS [United States Citizenship and Immigration Services] officer and ensures it really exists," he said. "I think USCIS went overboard with the 2011 regulations."

Stock said most R-1 cases he works on pertain to Christian denominations, such as Roman Catholic religious orders transferring priests and nuns to the U.S., Pentecostal church planters from Africa and Brazil, Mormon missionaries and workers, Korean Presbyterian ministers and Methodist ministers from India.

However, a wide variety of religions are eligible.

William Cocks, a spokesperson for the State Department's Bureau of Consular Affairs, said the vetting process for the program has become particularly thorough.

"National security is our top priority when adjudicating visa applications. Every prospective traveler to the U.S. undergoes extensive security screening. Applicants are continuously screened, both at the time of their visa application and afterwards, to ensure they remain eligible," he said. "We are constantly working to find mechanisms to improve our screening processes."

Others well-versed in the R visa program say it is no longer necessary – not because of security concerns but for the sheer fact that resources ought to be focused on those already in the U.S, not in bringing outsiders in.

"We need to take care of our needs here. We don't see the benefit in bringing over foreigners," said reserve sheriff and former Los Angeles Police Department Islamic chaplain Sheik Qazi Asad. He also said that when R-1 holders do come to the U.S., their employers do their own second-tier of vetting to ensure applicants have not spewed any radical rhetoric and discourage permanent stays. "We make sure they have return tickets," he said.

Rep. Steve King, R-Iowa, a staunch critic of the R-visa program, in 2015 introduced the Religious Worker Visa Reciprocity Act, which would narrowed the scope of the visa. But his proposal has failed to pass.

"The United States has the most generous immigration policy in the world. However, the generosity is being repaid with fraud and abuse," he stated at the time. "This is a sensible opportunity to narrow the R visa and make a stand for religious freedom. All U.S. religious workers should have the same access to a foreign country as that country's religious workers have to the U.S. This is a sensible solution that both limits waste, fraud and abuse by lowering the number of considered petitions and promoting religious freedom."

DHS and the White House did not respond to a request for comment.

But those in favor of the visa say it has cultivated and spread piety all across the country.

Rev. Ruben Duran, director for new congregations at the Evangelical Lutheran Church in America (ELCA) called the R-1 a “wonderful vehicle” to help bring foreign leaders to needed churches, and said that they take extreme care in selecting the appropriate candidates, with the majority of their R-1 holders coming from Asia, South America and India.

Patricia Zapor, communications director of the Catholic Legal Immigration Network (CLINIC), said the program is “very important” in enabling those in religious vocations to serve their communities.

“Without their efforts and presence many vulnerable adults, children, elderly would face additional daily struggles and societal problems – poverty, addiction, abuse,” she added. “It allows men and women in the U.S. to exercise their freedom of religion by having enough priests in the U.S. to meet the needs of the Catholic population.”