By Roberta Rampton

WASHINGTON, April 14 (Reuters) - U.S. Vice President Mike Pence will travel to South Korea on Sunday in what his aides said was a sign of the U.S. commitment to its ally in the face of rising tensions over North Korea's nuclear program.

Pence's Seoul stop kicks off a long-planned 10-day trip to Asia - his first as vice president - and comes amid concerns that Pyongyang could soon conduct its sixth nuclear test. President Donald Trump has warned against further provocations, sending an aircraft carrier group to the region as a show of force. His officials have been assessing tougher economic sanctions as well as military options to curb North Korea's nuclear ambitions.

Pence plans to celebrate Easter with U.S. and Korean troops on Sunday before talks on Monday with acting President Hwang Kyo-ahn.

"We're going to consult with the Republic of Korea on North Korea's efforts to advance its ballistic missile and its nuclear program," a White House foreign policy adviser told reporters, previewing Pence's trip.

Pence will land in Seoul the day after North Korea's biggest national day, the "Day of the Sun." The White House has contingency plans for Pence's trip should it coincide with a another North Korean nuclear test by its leader Kim Jong Un, the adviser said.

"Unfortunately, it's not a new surprise for us. He continues to develop this program, he continues to launch missiles into the Sea of Japan," the adviser said.

"With the regime it's not a matter of if - it's when. We are well prepared to counter that," the adviser said.

'FREE AND FAIR' TRADE

Pence expects to talk about the "belligerence" of North Korea at stops in Tokyo, Jakarta and Sydney, the White House adviser said.

But the need for "free and fair trade" will also be a theme, the adviser said.

Trump campaigned on an "America First" trade policy, complaining that trade partners in Asia and elsewhere had taken advantage of the United States.

One of his first acts in office was to remove the United States from the 12-nation Trans-Pacific Partnership (TPP) trade deal negotiated by former President Barack Obama.

"Withdrawing from the TPP shouldn't be seen as a retreat from the region. On the contrary, our economic presence in the region is enduring," the adviser said.

On Tuesday, Pence will kick off economic talks with Japan requested by Trump and Prime Minister Shinzo Abe. The discussions will focus more on setting a "framework" for future talks rather than on specific industry issues, a White House official said.

Pence will meet with business leaders at each stop, including in Jakarta, though he was not expected to wade into the weedy details of disputes between the Indonesian government and U.S. companies like mining giant Freeport-McMoRan Inc .

"We're going to discuss the business environment in Indonesia in a general sense," a White House official said. (Reporting by Roberta Rampton; Editing by Michael Perry)