A bipartisan group of House members are pressing President Donald Trump to take concrete steps to secure the unconditional, safe return of several U.S. citizens and permanent residents imprisoned in Iran after the administration leveled sanctions last month on its prison system and officials who run it.

Reps. Ileana Ros-Lehtinen (R., Fla.), Ted Poe (R., Texas), Nita Lowey (D., N.Y.), and Ted Deutch (D., Fla.), this week introduced a bipartisan resolution calling on Trump to make the release of at least six U.S. citizens and permanent legal residents the "highest of priorities" and urging the U.S. and its allies who also have citizens detained in Iran to create a multi-national task force to secure their release.

Ros-Lehtinen blamed the Obama administration for giving Tehran an incentive to take more U.S. hostages with its $1.7 billion payment to Tehran last year, $400 million of which was paid in cash and timed to ensure the release of four U.S. hostages.

"The obvious result of paying a rogue regime a ransom for the return of hostages was that it would incentivize the taking of even more hostages," Ros-Lehtinen said Thursday. "And that is precisely what happened when Iran was paid a $1.7 billion ransom for American hostages."

Since the U.S. payments were made early last year, Iran's Revolutionary Guard Corps has taken hostage Siamak and Baquer Namazi, Karan Vafadari and at least one other American citizen whose family does not want him named, as well as permanent legal residents Nizar Zakka and Afarin Niasari. The Iranians have sentenced them on false espionage and other charges, according to their lawyers and families.

In recent months, Iran also has imprisoned several dual-nationals of other western nations, including the United Kingdom, France, Germany and Canada.

The Obama administration has argued that the payment was unrelated to the American hostages' release, and was paid to resolve a longstanding dispute over money Iran had paid to the U.S. to buy jets, a deal that never happened after the Shah of Iran was overthrown in 1979. However, officials have admitted that the $400 million payment was delayed to gain leverage in the hostage negotiations, and Republicans have warned that decision would lead to more hostage situations.

Nizar Zakka, a Lebanese citizen and permanent resident of the United States, said through his attorney last year that Iranian officials in April told him it would take as much as $2 billion to ensure his release from captivity. In September, Iranian officials lowered that amount to $4 million, and told him that he was spared the death penalty but would remain in prison for 10 years until the payments is made.

In calling for an international task force to try to negotiate the prisoners' release, Ros-Lehtinen said it is time for the U.S. and its allies to "send a strong message to the regime in Tehran that this tactic will not pay off—the regime must release all hostages unconditionally."

The resolution also calls for the safe return of Robert Levinson, a former Federal Bureau of Investigation agent who went missing in Iran in 2007. After the prisoner exchange early last year, then-Secretary of State John Kerry said the Iranian government had pledged to help in the search for Levinson.

Even though the Trump administration has told Congress that Iran is complying with the terms of the nuclear deal, Secretary of State Rex Tillerson has questioned whether the U.S. and its allies should continue lifting sanctions given Iran' continued support for terrorism.

During last week's follow-up talks with Iranian officials on the implementation of the nuclear deal in Vienna, the first during the Trump administration, State Department officials raised concerns regarding the cases of U.S. citizens detained and missing in Iran and pushed for their release.

Last month, the administration leveled new economic sanctions against senior Iranian officials and its prison systems for what White House officials said were widespread human rights abuses, including systematic torture of those being held.

The sanctions target Tehran Prisons Organization and Sohrab Suleimani, a senior official in the prison system.

Sohrab Suleimani is responsible for running Iran's Evin Prison, which is known for its harsh conditions and forced interrogations. Many, if not all, of the prisoners with U.S. citizenship or green cards are being held there.