WASHINGTON/MEXICO CITY (Reuters) - U.S. President Donald Trump’s top trade adviser expressed optimism on Tuesday about reaching agreement on a revised free trade pact with South Korea, days after Trump suggested scrapping the deal with a key American ally.

Senior U.S. lawmakers and America’s biggest business lobby urged Trump not to pull out of the five-year-old U.S.-South Korea Free Trade Agreement (KORUS), especially at a time of heightened tensions over North Korea’s nuclear missile tests.

U.S. Trade Representative Robert Lighthizer, speaking in Mexico City after a second round of NAFTA talks with Canada and Mexico, said negotiations with Seoul were continuing.

“We have a negotiation we’re in,” Lighthizer told reporters when asked whether KORUS would be terminated. “My hope is that we’ll have a successful discussion with the Koreans as things proceed and that the problems with that agreement from our perspective will be worked out.”

Trump said on Saturday he would discuss KORUS’s fate with advisers this week, prompting widespread concern among lawmakers and the business community.

The chairmen and senior Democrats on the House of Representatives Ways and Means Committee and the Senate Finance Committee said in a statement on Tuesday that North Korea’s sixth and largest nuclear bomb test on Sunday “underscores the vital importance of the strong alliance between the United States and South Korea.”

The statement by House Ways and Means Committee Chairman Kevin Brady, senior Democrat Richard Neal and Senate Finance Committee Chairman Orrin Hatch and senior Democrat Ron Wyden said talks to improve South Korea’s implementation and compliance with the trade agreement were welcome. But it said the agreement itself was central to the U.S.-South Korean alliance.

In a separate letter to Trump, Senator Joni Ernest, a Republican from Iowa in the U.S. corn belt, said the South Korean market was especially important for U.S. beef, corn and pork producers.

“Terminating KORUS would leave our farmers at a competitive disadvantage to those in other countries that enjoy preferential trade access to Korea,” Ernst wrote.

In a strongly worded statement the president of the U.S. Chamber of Commerce, which represents more than 3 million businesses, also opposed any “rash and irresponsible” withdrawal.

“We do not believe this move would create a single American job — but it would cost many,” said Tom Donohue, who warned that it would damage relations between the White House and business community.

“Ironically, states across mid-America that voted for the president would take the hit from withdrawal as their agricultural and manufactured goods exports fell in the wake of such a move,” Donohue said.