KABUL, Afghanistan — The departing head of the United Nations mission in Afghanistan, Kai Eide, warned Thursday that the window to achieve success in Afghanistan was narrowing and that “negative trends” in the country could become irreversible over the next year.

Speaking at his final news conference as the United Nations special representative in Afghanistan, Mr. Eide stressed the need for a political solution to end the conflict with the Taliban.

He also cautioned against excessive militarization of international efforts in Afghanistan, a long-standing concern that has taken on greater significance as the American-led military operation grows and includes more nation-building.

Mr. Eide also warned that military operations against insurgents needed to be waged in a manner that did not impede any political processes.

Mr. Eide’s comments came as thousands of troops from the United States, Britain and Afghanistan began to work to restore civilian services after finishing the huge combat phase of its effort to retake a Taliban stronghold at Marja in the southern Helmand Province.

Mr. Eide, a Norwegian diplomat, announced late last year that he would step down from his post in Afghanistan when his contract expired. His one-and-a-half year tenure was marked by rising bloodshed and criticism of his handling of the fraud-plagued presidential elections in August.

His former deputy, the American diplomat Peter W. Galbraith, accused Mr. Eide of covering up fraud that aided President Hamid Karzai. Mr. Eide vehemently denied the allegations and said he had followed Afghan law. Mr. Galbraith was dismissed last autumn.

On Thursday, Mr. Eide said he would spend his few remaining days in Afghanistan pressing Mr. Karzai over a much-criticized maneuver in which the Afghan president gave himself the authority to name the five members of an electoral monitoring commission. The move would sap United Nations oversight of future elections, and could undermine their credibility in the eyes of Afghans and the international community.

Mr. Eide said he had made “some progress” with Mr. Karzai in negotiations over the panel, the Electoral Complaint Commission, but offered no details. Previously, the United Nations had seated foreigners to three of the seats of the commission, with the other two held by Afghans.

Mr. Eide, who is set to leave Afghanistan on Sunday, said 2010 would be the “most challenging” year since the American-led invasion of Afghanistan toppled the Taliban in 2001, and it would be a critical period for the international community’s efforts in Afghanistan.

“It is a year where negative trends have to be reversed or they will become irreversible,” he said, adding, “Decisive success within a year or two in a country marred by conflict is unachievable.”

Last year was the deadliest since 2001 for members of the NATO-led military coalition and Afghan civilians. Saying that “clocks in foreign countries tick faster” than change can occur in Afghanistan, Mr. Eide called for international patience with the slow pace of progress. He said that any resolution to the conflict needed to balance military and political approaches.

“I believe that a political process is indispensable for finding a solution to this conflict,” he said. “I believe the focus is too much on military side and too little on political side and civilian side, and our strategy has unfortunately been too much military driven.”

In his time in Kabul, Mr. Eide tried to carve out a more important role for the United Nations mission, aiming to coordinate international assistance and act as the main international interlocutor with the Afghan government. He also sought to distance his mission from NATO military operations and to assist the Afghan people more, taking a strong stand on investigating civilian casualties and human rights abuses.

Yet his efforts have not always succeeded, and the agency itself has become a target. Five United Nations staff members were killed when gunmen attacked a guesthouse in central Kabul last year, just days before a second round of elections.