Story highlights Up to 10,800 U.S. troops will remain in Afghanistan at the start of 2015

Hagel's unannounced visit will include other stops in the region

The trip follows a recent announcement that he will step down as defense chief

He has served as defense secretary since last year

The U.S. will keep a larger force in Afghanistan for the first few months of 2015 than it initially planned to, outgoing U.S. Defense Secretary Chuck Hagel said on Saturday.

During a joint press conference with Afghan President Ashraf Ghani , Hagel said that up to 10,800 troops will remain in the country at the start of next year; a previous announcement called for 1,000 fewer troops.

Hagel arrived in Afghanistan on Saturday in order to assess the nation as the United States begins the drawdown of its forces in the New Year, he said.

He said that President Barack Obama authorized the change in force size in order to provide military commanders "flexibility."

"The President's authorization will not change -- it will not change our troops' missions or the long-term timeline for our withdrawal," Hagel said.

Kabul has seen an uptick in Taliban attacks in recent weeks, a sign of instability that he said comes as no surprise.

"It's predictable that they would do everything they could and continue to do to try to disrupt and discourage the new government of President Ghani," he said.

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However, the spike in violence in the Afghan capital was not a factor in the decision to keep more U.S. troops in the country next year, a Defense Department official said. The decision is due to the late signing of the Bilateral Security Agreement, which allows a specified amount of U.S. troops to remain in Afghanistan after the combat mission ends this year.

Ghani's predecessor, Hamid Karzai , would not sign a security agreement with the United States.

Despite the attacks, the Pentagon said Afghan forces are performing "well," and are now in the lead on 99% of the missions.

"The gaps are in aviation and ISR (intelligence, surveillance, and reconnaissance)," a senior defense official told reporters on the flight to Afghanistan.

This is his fourth and last visit to Afghanistan as defense secretary.

His first trip to Afghanistan was 12 years ago as part of a Congressional delegation.

Asked by CNN how he would define victory more than a decade later Hagel said the people of Afghanistan are far better off today than they were 13 years ago, citing an elected government and a national security force.

"They're not completely there yet, but they've come a long way," Hagel said. "That's to the credit certainly of the United States, the sacrifice, the blood and the treasure that we've made there."

His visit comes after the establishment of a new unity government in the fall, which Hagel will express strong support for during his trip, a second senior defense official said.

The drawdown of U.S. forces in Afghanistan will be "gradual" compared with the withdrawal of troops from Iraq, the first official said.

In 2016, the number of troops will slide down to 5,500 and by 2017 the coalition will consolidate to Kabul.

"As we draw down the combat mission, we have not forgotten what brought America to Afghanistan over a decade ago," Hagel said on Saturday.

Maintaining a troop presence will allow the U.S. to monitor the progress of the Afghan security forces over time, according to the first official.

"The Afghan people and the Afghan government ...have asked us to stay," Hagel said contrasting the drawdown to the withdrawal troops in Iraq, where the U.S. could not negotiate an agreement that would allow a residual force in the country.

The trip to Kabul follows Kabul's announcement last month that he will step down as soon as the Senate confirms his successor. Obama announced Friday that Ashton Carter will be his nominee to replace Hagel.

Hagel, who has served as defense secretary since February last year, was forced out by Obama, several sources told CNN.

During remarks announcing his resignation, Obama praised him as an "exemplary" defense secretary, calling him critical to various national security accomplishments during his tenure.

He said Hagel's resignation was his own choice.

A critic of the Iraq war, Hagel took over from his predecessor Robert Gates to oversee the draw down from Afghanistan and a heavily cut Pentagon budget.

The former Nebraska senator was the last Republican in Obama's Cabinet. He is a Vietnam combat veteran.

Hagel's unannounced visit will include stops in other countries in the region.