WASHINGTON -- As attacks by the Nigerian militant group Boko Haram become increasingly violent, some members of Congress are criticizing the Obama administration’s refusal to formally designate the group a terrorist organization.

Senior U.S. officials contend that they still don’t know enough about Boko Haram and that the most serious attacks attributed to the group may not reflect its overall ideology. The designation would put the group on a watch list and would bar American individuals and entities from providing it with financial or other resources.

Johnnie Carson, the ranking U.S. diplomat for African affairs, has been quick to condemn the group’s actions. He noted that some of its members have been slapped with U.S. sanctions, but he argued before Congress recently that Boko Haram isn’t a homogenous organization.

“We believe it is an organization of several parts,” Carson told the House of Representatives Subcommittee on Africa, Global Health and Human Rights earlier this month. “We believe the individuals we have designated represent a core group which is desirous of attacking not only Nigerian targets of interest but also international targets of interest.”

Boko Haram, which means “Western education is sinful,” has targeted primarily Christians in an attempt to create an Islamist state governed by Shariah law in Africa’s most populous nation. The group claimed responsibility for a series of attacks in December that targeted churches across the country and claimed the lives of dozens of Nigerians.

To some critics, Carson’s explanation doesn’t stand up against the group’s increasingly militant efforts.

“The U.S. government has a lot to learn about Boko Haram and their intelligence should remain high, but that should not be an excuse for inaction,” said Morgan Roach, a researcher at the Heritage Foundation, a conservative research center in Washington.

Rep. Patrick Meehan, R-Pa., introduced a resolution in the House in May urging the State Department either to designate Boko Haram a terrorist organization or explain its reasons for not doing so. He introduced the resolution in response to the group’s increasing aggression, including the Christmas bombings and an attack on the United Nations offices in Nigeria’s capital, Abuja, he said.

“We’re seeing a growing body of work, not a diminishing body of work, and I think the more those activities carry on the greater the demand will be for the Department of State to explain the position that they’re taking,” Meehan said.

Given the advancement in Boko Haram’s attacks, particularly its increased use of homemade bombs, some U.S. officials suspect that the group is receiving technical support and training from other extremist groups in the region. Gen. Carter Ham, the commander of the U.S. military’s Africa Command, and Director of National Intelligence James Clapper have explicitly said that Boko Haram now is allied with two other major African militant groups, al Qaida in the Islamic Maghreb and Somalia’s al Shabab, both of which the State Department has designated as terrorist organizations.

“I think (given) the amount of training and the quickness with which Boko Haram has been able to improve their tactics, improve their attacks, we have to be looking a bit more closely at where this is coming from,” Roach said.