NEW YORK, July 26 (Reuters) - Afghanistan needs more mobile foreign troops as it suffers its bloodiest phase in nearly five years, with militants gearing up to test NATO-led forces, the country's ambassador to the United States said on Wednesday.

Said T. Jawad said Afghanistan had come a long way since it became the front line for the war on terrorism when U.S.-led forces ousted the ruling Taliban and began to root out Osama bin Laden's al Qaeda group, blamed for the Sept. 11 attacks.

"But we are not out of the woods," Jawad told Reuters ahead of a speech to New York's Afghan community on Thursday. "We are facing challenges. Afghan people are determined to win this war, but we are facing some serious bumps on the road."

"What we need is to have more mobile and agile international forces to respond very quickly to the daily attacks of terrorists," he said, adding that more investment was also needed in the reconstruction of the country.

A NATO-led international force is due to expand military operations into southern Afghanistan and take over command in the area from the United States on Monday, extending its control over security to all of the country except the east.

The coalition force in Afghanistan says it has killed more than 600 militants since the start of June. Most of the militant attacks have been occurring in the south, near the border with Pakistan.

"We see Taliban coming to Afghanistan in large numbers and there are a variety of reasons why we are witnessing that spike in terrorist activities", Jawad said.

"(The NATO forces) show the consensus of the international community on the need to stay focused and help Afghanistan, but also provide an opportunity for terrorists to attack Afghan and international forces and test the military might of NATO."

Jawad said the spike in violence could also be blamed on the inability of the Afghan police and army to combat the problem and on Afghanistan's location in a "tense" region.

"The terrorists are still able to have access to training grounds, financial resources and ideological safe havens outside Afghanistan borders", he said, adding that he does not believe Osama bin Laden is hiding in Afghanistan.

But while Afghanistan was still facing serious challenges, Jawad said the country's double-digit economic growth, the enthusiasm of the people in electing a president, the return of millions of children to school and the homecoming of millions of Afghan refugees showed how far the nation had come.

"I cannot wait to see what we will achieve within in a generation," he said.

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