(CNSNews.com) – The Obama administration has designated as terrorists three senior figures in Boko Haram, the Islamist group responsible for deadly attacks targeting Christians in Nigeria, but two senior Republican lawmakers Thursday repeated earlier calls for the group to be listed as a foreign terrorist organization.

Boko Haram terrorists on Sunday killed around 16 people and wounded scores more in suicide bombings at three churches in the north of Africa’s most populous country. It was the third consecutive Sunday that saw church attendees killed at the hands of the group, whose initial campaign promoting shari’a and opposing “Western education” has increasingly taken the form of an anti-Christian jihad.

On the previous Sunday, June 10, at least six people were killed and dozens injured when gunmen opened fire at a church, while on June 3 at least 15 people were killed and 42 wounded in a suicide bombing at a church in Bauchi state, also in the north.

On Thursday, the State Department announced it was designating three Boko Haram leaders as “specially designated global terrorists” (SDGTs) under executive order 13224, a post-9/11 mechanism designed to disrupt funding to terrorists.

Americans are prohibited from engaging in transactions with SDGTs, and any assets they may have in the U.S. are frozen.

The three Nigerians are Boko Haram’s “most visible leader,” Abubakar Shekau, and two men, Abubakar Adam Kambar and Khalid al-Barnawi, said by the department to “have ties to Boko Haram and have close links to al-Qaeda in the Islamic Maghreb, a designated Foreign Terrorist Organization.”

“Under Shekau’s leadership, Boko Haram has claimed responsibility for numerous attacks in northern Nigeria, its primary area of operation,” the State Department said in a statement. “In the last 18 months, Boko Haram or associated militants have killed more than 1,000 people.”

The statement referred to three particular attacks – a suicide bombing at the U.N. headquarters in Abuja last August that cost 25 lives; an attack on a Catholic Church last Christmas Day that killed at least 35 people; and what is believed to be the group’s deadliest assault to date, a series of attacks in Kano last January 20 that killed more than 180 people.

The State Department said the designations, taken in consultation with the U.S. Treasury and Justice Department, “demonstrate the United States’ resolve in diminishing the capacity of Boko Haram to execute violent attacks.”

But Rep. Peter King (R-N.Y.), chairman of the Homeland Security Committee, and Rep. Patrick Meehan (R-Pa.), chairman of the committee’s counterterrorism and intelligence subcommittee, called the step “insufficient.”

“The legal ramifications of this designation only affect dealings with three designated individuals, and not the wider Boko Haram organization, which is growing in intent, capability and targeting capacity,” they said in a statement.

King and Meehan pointed out that Thursday’s action does not provide for the Treasury to sanction additional members of Boko Haram, so for any others to be penalized they would have to be found to have a proven relationship to Shekau, Kambar or al-Barnawi.

“If these three individuals are killed, the designations are meaningless.”

The lawmakers repeated earlier calls for Secretary of State Hillary Clinton to designate Boko Haram as a foreign terrorist organization (FTO).

“The Department of State refuses to explain its rationale for not designating Boko Haram an FTO, which puts American lives at risk,” they said. “Given Boko Haram’s trajectory and intent to carry out terrorist attacks against Western targets, including possibly the Homeland, we must take the growing threat seriously.”

Wisdom of designation debated

This is the third time this year King and Meehan have made the FTO appeal.

In a request to Clinton at the end of March, they cited a bipartisan report the committee first released last November entitled “Boko Haram: Emerging Threat to the U.S. Homeland.”

In a second appeal to Clinton, last month, King and Meehan cited media reports saying the Justice Department believes Boko Haram meets the legal requirements for FTO designation.

“It is deeply frustrating and concerning that a formal request from the Department of Justice for FTO designation of Boko Haram has gone unheeded for almost six months,” they wrote in the May 18 letter.

“Designating Boko Haram an FTO is essential to giving our intelligence and law enforcement agencies the legal authorities to deter individuals who might be providing support to Boko Haram in the U.S. and abroad, and freeze any known Boko Haram assets,” they said. “FTO designation can no longer wait. We urge you to act immediately.”

Not everyone agrees. A group of U.S. scholars with Africa expertise warmed Clinton in a letter last month that designating the Nigerian group as an FTO “would internationalize Boko Haram’s standing and enhance its status among radical organizations elsewhere.”

The signatories also noted that designation would make it illegal for non-governmental organizations to interact with the group’s members, “even if the purpose of such contact was to persuade them to renounce violence.”

In that context they pointed to former President Jimmy Carter’s response to a July 2010 Supreme Court ruling upholding prohibitions on “material support” to terrorist groups. Carter said at the time the ruling could criminalize his Carter Center’s “work to promote peace and freedom.”