Washington: Senate Republican leader Mitch McConnell said on Monday he opposed US President Barack Obama's plans to normalize relations with Cuba, and spoke of steps lawmakers might take to rein in the new policy.

Interviewed by Reuters, McConnell also said that he and the president, a Democrat, had discussed possible major tax reform legislation and that any effort should not focus on the country's biggest corporations alone, but also include help for small businesses.

On another international matter McConnell, who takes over in January as Senate majority leader, said North Korea's computer hacking of Sony Corp was more serious than an act of vandalism, taking issue with a characterization Obama had used to describe the cyberattack. McConnell declined to spell out steps he thought the United States should take in response.

"This is a serous threat to the United States," he said.

Speaking by telephone from his home state of Kentucky, McConnell said he agreed with the Senate's most outspoken critics of Obama's new Cuba policy, Republican Senator Marco Rubio of Florida and Democratic Senator Robert Menendez of New Jersey, "that it was a mistake."

Obama's actions to forge relations and expand commercial ties with the Communist-led island after half a century of hostility has divided Republicans in Congress and could weigh on the 2016 campaign for president.

McConnell said there were some "pretty obvious" ways to keep the policy from being fully implemented. Only Congress has the power to remove some barriers to relations with Cuba since "a number of sanctions" were written into law, he said. He said any U.S. ambassador to Cuba would require Senate approval.

"Look at Vietnam," McConnell said. "We normalized relations with them and they are a Communist regime that still represses people. Sometimes engagement works, sometimes it doesn't."

With many Republicans and Democrats backing tax reform, it could become an issue in the last two years of Obama's presidency. The last major reform was in 1986.

While McConnell repeatedly referred to "comprehensive" legislation, he also stressed that such a measure had yet to take shape and would require bipartisan agreement.

In pressing for broad reforms that include small businesses, McConnell said, "It's pretty hard to argue it's a good idea to take Fortune 500 companies down to 25 percent (tax rate) and leave a mom-and-pop operation in Louisville in the high 30s."