By David Ljunggren and Roberta Rampton

WASHINGTON (Reuters) - U.S. President Donald Trump said on Thursday he was prepared to raise the case of two Canadians detained by Beijing with Chinese President Xi Jinping, an act that could potentially ease a major dispute between Canada and China.

Trump made his comments before a meeting with Canadian Prime Minister Justin Trudeau in the White House. Trudeau said later the two men spent considerable time discussing China, adding he expected Trump and Xi to discuss the matter on the sidelines of a Group of 20 summit in Japan next week.

Trudeau wants Washington to do more to solve a crisis that erupted after Canadian police arrested an executive of Chinese firm Huawei Technologies Co Ltd on a U.S. warrant in Vancouver last December.

Trump is due to meet Xi to discuss the trade war between the two countries and has suggested Huawei could be part of an eventual deal. China has charged the two Canadians with spying and blocked imports of canola seed and pork from Canada.

"Anything I can do to help Canada I will be doing," Trump said when asked whether he would raise the matter with Xi.

Asked again, he replied: "I would at Justin's request, I will absolutely."

Trudeau later sidestepped reporters' questions about whether he had specifically asked Trump to raise the case with Xi.

"We know the president will have a meeting with President Xi and we absolutely expect the subject of the Canadians who have been arbitrarily and unjustly detained in China will be on the agenda," he told reporters.

In Beijing, Chinese Foreign Ministry spokesman Lu Kang said Canada was responsible for the problems between their two countries, and Canada is "very clear" about the reasons for this and knows what it has to do.

"Canada, or any other country, should respect China's judicial sovereignty and should not make thoughtless remarks or gesticulate about things that fall under China's judicial sovereignty," he told reporters.

A senior Canadian government source told Reuters that allies around the world had been pressing Beijing on the matter and said Ottawa did not want to be too specific in public about its tactics, citing concerns for the two men.

TRADE PACT

The comments by Trump - who also called Trudeau "a good friend of mine" - underlined the steady improvement in the relations between the two men since June 2018, when he labeled the Canadian leader weak and dishonest.

Another major trade challenge facing Trump is ratification of a new trade pact he signed last year with Mexico and Canada, designed to replace the North American Free Trade Agreement. It is crucial to Mexico and Canada, which both send more than 75 percent of their goods exports to the United States.

The deal was negotiated in 2017 and 2018 after Trump repeatedly threatened to withdraw from NAFTA if he could not get a better trade agreement for the United States.

Mexico's Senate on Wednesday approved the new deal, known as the U.S.-Mexico-Canada Agreement (USMCA), and Trudeau says Canada is ready to follow suit.

Trump's fellow Republicans in the U.S. Congress want to pass the deal by the end of August, lest it get mired by politics in the run-up to the presidential and congressional elections in November 2020.

But the pact's ratification by U.S. lawmakers is uncertain because Democrats, who control the House of Representatives, are seeking tougher enforcement measures.

U.S. House Speaker Nancy Pelosi on Wednesday outlined impediments to congressional approval, including concerns over labor and environmental protections.

"I really believe Nancy Pelosi and the House will approve it, I think the Senate will approve it rapidly," Trump said. Republicans are the majority party in the Senate.

Trudeau later met Pelosi and said the two had a good talk on the treaty during which he "offered to be helpful in responding to or allaying certain fears." Canada would not get involved in the U.S. ratification process, he added.

The United States last month lifted steel and aluminum tariffs it had imposed on Canada, partly because Ottawa promised to do more to stop transshipments of Chinese steel and aluminum entering the U.S. market via Canada.

(Reporting by David Ljunggren and Roberta Rampton; Additional reporting by Susan Cornwell in Washington, Steve Scherer in Ottawa and Ben Blanchard in Beijing; Editing by Grant McCool and Peter Cooney)