Secretary of State Hillary Rodham Clinton arrived in the Afghan capital Wednesday for an unannounced visit intended to boost faltering reconciliation efforts ahead of the planned withdrawal of NATO forces.

Clinton, who traveled to Kabul after visits to Libya and Oman, was scheduled to meet Thursday with President Hamid Karzai and other government and parliamentary leaders. Her trip comes at a time of increased tensions between U.S. and Afghan officials over how to pursue peace with the radical Islamist Taliban movement after a decade-long insurgency.

The Afghan government’s reconciliation efforts were dealt a setback last month when a suicide bomber assassinated Burhanuddin Rabbani, the leader of Afghanistan’s High Peace Council and the government’s point man in talks with the Taliban. Rabbani, a former president who led the Northern Alliance during Afghanistan’s civil war in the 1990s, had been tasked with brokering a peace deal with the Taliban. He was killed at his home by a man who concealed a bomb under his turban while posing as a Taliban peace emissary. His death has spread despair among some Afghan officials over whether an accord with the Taliban is possible.

Aides to Clinton said the visit will “signal U.S. support for a secure, stable Afghanistan” despite Rabbani's killing and a recent high-profile attack on the U.S. Embassy compound in Kabul.

“We want to emphasize that the United States remains committed to Afghan reconciliation and will support President Karzai and his efforts,” said a senior State Department official who traveled with Clinton to Afghanistan for the meetings.

U.S. officials are pushing for a negotiated settlement with the Taliban as a crucial step toward ending the conflict and have engaged in secret parallel talks with Taliban leaders, so far without success.

Karzai, who has criticized the secret U.S. talks, has urged a greater role for Pakistan in the reconciliation process, noting that many of the key Taliban commanders use Pakistan’s lawless tribal region as a base. The State Department official, who spoke on the condition of anonymity to discuss sensitive diplomatic matters, said Clinton “agrees with President Karzai that Pakistani cooperation is critical.”

Clinton also will seek to nudge Karzai on a proposed bilateral agreement on security cooperation that would take effect after the withdrawal of most U.S. forces, aides said. The Obama administration plans to withdraw by late next year the 33,000 “surge” troops that were deployed to Afghanistan in 2010 and to turn over security for most of the country to the Kabul government by 2014.

The Associated Press reported from Islamabad that Clinton was expected to arrive in the Pakistani capital later Thursday, and would be joined by CIA chief David Petraeus and some senior U.S. military officials. She is likely to press the Pakistani leadership to do more to stop infiltration of militants across the border into Afghanistan.

Clinton landed in Kabul after visiting the sultan of Oman earlier Wednesday to thank him for helping to secure the release of American hikers from Iran last month and to appeal to Arabs to further increase pressure on Iran.

In Oman, Clinton held talks with Sultan Qaboos bin Said in his palace in the capital of the Arabian Peninsula country. The meeting came a week after revelations of an alleged Iranian plot to kill an ambassador from Saudi Arabia, Oman’s neighbor and close ally.

The Obama administration is seeking to use the plot to build support from Arab governments for further isolating Iran diplomatically and economically.

“We’re sending out worldwide messages now that are tailored to each country,” a senior State Department official told reporters traveling with Clinton, “asking them to use their relations with Iran in way to really focus the Iranians on the risks they face based on this type of behavior.”

Clinton arrived at the elaborately tiled Omani palace Wednesday morning, a day after meeting with leaders of Libya’s transitional government in Tripoli, and spoke privately with the sultan.

Oman, a member of the six-nation Gulf Cooperation Council, traditionally maintains good relations with Iran and has helped mediate conflicts with the Islamic Republic in the past. Omani officials interceded with Iranian leaders last year to secure the release of American hiker Sarah Shourd, and were influential last month in persuading Iran to free Joshua Fattal and Shane Bauer . The three were backpacking in an unmarked area along the Iran-Iraq border in July 2009 when Iranian security officials arrested them on suspicion of spying.

Clinton thanked Qaboos on behalf of the administration and discussed how Gulf countries could use their influence to dissuade Iran from seeking nuclear weapons and supporting terrorist groups.

Said the senior State Department official, who spoke on the condition of anonymity in keeping with diplomatic protocol: “We hope the Omanis will use their relations with Iran, as they have in the past, to help the Iranians understand the risks of what they’re doing.”

Staff writer William Branigin in Washington contributed to this report.