Washington (CNN) -- The Obama administration strongly defended its handling of the Libyan crisis Thursday, drawing a clear line between military and political objectives while dismissing criticism that it has failed to adequately consult with members of Congress.

"We are not engaged in militarily-driven regime change," White House Press Secretary Jay Carney told reporters. Instead, the administration is engaged in "time-limited, scope-limited" action with other countries to protect civilians from forces loyal to strongman Moammar Gadhafi.

American forces will be transitioning to a "support and assist" role in the international coalition within a matter of days, he promised. U.S. ground troops will not be sent into Libya, he stressed.

Later in the day, NATO Secretary-General Anders Fogh Rasmussen announced that NATO members will take over enforcement of the no-fly zone as soon as Sunday. But he stopped short of interpreting that mandate as a license to attack pro-Gadhafi forces who may be threatening unarmed civilians. Nor did he say its goal was to effect regime change.

President Barack Obama has said the administration's ultimate objective is Gadhafi's removal from power. U.S. officials have indicated they hope Gadhafi will be removed quickly by forces currently loyal to him, though they haven't publicly called for a coup.

Carney listed a series of recent meetings, hearings and briefings by top officials -- including the president -- with members of Congress on Libya. The list was produced in response to accusations by House Speaker John Boehner, R-Ohio, and others that the White House failed to properly consult with legislators before launching the Libyan mission.

Boehner sent a letter to the president Wednesday complaining that "military resources were committed to war without clearly defining for the American people, the Congress, and our troops what the mission in Libya is and what America's role is in achieving that mission."

Carney said the administration has "endeavored to answer (Boehner's) questions already," and noted that the speaker received a classified intelligence briefing on March 14. He also accused some critics of being "perhaps driven by politics."

Carney said the president will continue to speak out on Libya "with relative frequency."

A Republican source, meanwhile, told CNN that Secretary of State Hillary Clinton, Defense Secretary Robert Gates, Joint Chiefs of Staff Chairman Adm. Mike Mullen, and National Intelligence Director James Clapper will deliver a classified briefing to members of Congress on March 30.

Hearings were also scheduled for the House Foreign Affairs and House Armed Services committees next week, according to a Democratic source.

Asked about the financial cost of the mission, Carney said expenses should be covered by existing Pentagon funds. But he told reporters, "I don't want to get into numbers. I'm not an economist."

While the White House insists it has been -- and will continue to be -- responsive to questions about the Libyan mission, critics on the right and the left remain unmollified.

Shortly after Carney spoke to reporters, Brendan Buck, a spokesman for Boehner, released a statement arguing that "it's fair to say the (speaker's) letter wouldn't have been necessary if the speaker -- and the American people -- had received answers."

Buck also said it's "important to note that an update on conditions in Libya is far different than a briefing on military operations under consideration."

One key House Republican called Wednesday for a withdrawal of U.S. forces, arguing that Obama had failed to rally public support for military action.

"Mr. President, you have failed to state a clear and convincing explanation of the vital national interest at stake, which demands our intervention in Libya," said Rep. Candice Miller, R-Michigan. "You have failed to state a clearly defined mission for our military to defend that interest. ... I believe you must pull our forces from the coalition immediately."

Rep. Tom McClintock, R-California, contends the president violated the 1973 War Powers Act and other constitutional restrictions on authorizing military action.

The president's actions constitute "a usurpation of constitutional powers clearly and solely vested in the United States Congress and is accordingly unlawful," McClintock said Wednesday.

Some liberal Democrats have also expressed unease with the intervention, particularly in regard to the relative lack of congressional consultation and the prospects for an open-ended conflict.

Arizona Rep. Raul Grijalva and California Reps. Barbara Lee, Mike Honda and Lynn Woolsey released a statement earlier in the week arguing that "the United States must immediately shift to end the bombing in Libya."

"We will fight in Congress to ensure the United States does not become embroiled in yet another destabilizing military quagmire in Libya with no clear exit plan or diplomatic strategy for peace," they said.

Top Senate Democrats, however, continue to defend the administration, insisting that Obama moved methodically and carefully to assemble a strong international coalition capable of saving innocent lives and reinforcing the broader Middle East reform movement.

Some analysts have echoed complaints about what they insist was unclear administration guidance about ultimate U.S. goals in Libya and the methods being used in pursuit of those objectives.

Obama has been "fairly muddy in what he's said," Max Boot, a senior fellow at the Council on Foreign Relations, argued Wednesday. The president has been "reacting frantically" to events and "being pulled hither and yon."

Boot predicted air power would not be sufficient to knock out the Gadhafi regime, and warned of a "protracted and costly stalemate" if the United States doesn't send in military advisers to help arm and train the rebels.

Obama may be hoping for a palace coup, he said, but "I wouldn't bet on it."

Boot also stressed the need for more planning for a post-Gadhafi Libya. There's a "real danger of chaos" and protracted tribal warfare if Gadhafi falls, he said. Al Qaeda may be able to exploit such a situation, he warned.

He blasted the White House for "not really preparing the American people for the possibility that this could be a protracted and expensive conflict."

"The public and the administration should not be going into this with rose-colored blinkers on," Boot said.

CNN's Brianna Keilar, Elise Labott, and Deirdre Walsh contributed to this report