WASHINGTON, Dec 26 (Reuters) - The White House on Friday urged India and Pakistan to avoid actions that increase tensions between them following an attack last month in Mumbai by suspected Islamist militants that killed 179 people.

The U.S. appeal for calm came after India warned its citizens it was unsafe to travel to Pakistan and Islamabad canceled army leave and moved some of its troops from its western border.

The back-and-forth moves marked a dramatic rise in tension between the nuclear-armed neighbors after last month's attack in Mumbai, India's financial capital, by suspected Islamist militants that New Delhi charged were based in Pakistan.

U.S. National Security Council spokesman Gordon Johndroe said the White House had seen reports about both sides' actions and was staying in touch with its embassies in the region.

"We hope that both sides will avoid taking steps that will unnecessarily raise tensions during these already tense times," Johndroe said.

"We continue to be in close contact with both countries to urge closer cooperation in investigating the Mumbai attacks and in fighting terrorism generally," he said. (Reporting by David Alexander; editing by Mohammad Zargham)

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