A U.S. senator wants his government to hold up a Chinese state-owned company's $15.1-billion takeover of Calgary-based Nexen Inc. as a means to press China on its trade policy as the U.S. securities regulator also looks into allegations of insider trading surrounding the deal.

Charles Schumer made his argument in a letter sent Friday to Treasury Secretary Timothy Geithner, who chairs the Committee on Foreign Investment in the United States, or CFIUS, a body that reviews foreign investments in U.S. companies.

The New York Democrat wrote that he "sincerely" hopes the friendly deal between China National Offshore Oil Co. and Nexen (TSX:NXY) deal goes through and that Chinese companies continue to invest in the United States.

"But I urge you not to miss this opportunity -- the largest foreign acquisition ever by a Chinese company -- to hold China to the commitments it has made to provide a level playing field for U.S. companies seeking to access Chinese markets."

Although Nexen (TSX:NXY) is based in Canada, it has offshore holdings in the U.S. Gulf of Mexico and CNOOC said the deal requires approval from regulators in the U.S. Schumer says he expects that means the deal will need the blessing of CFIUS to go ahead.

"While I support foreign investment in North America and the United States, and believe the proposed transaction will benefit the United States and help ensure the continued resurgence of our domestic energy sector, I believe we must take a long-term perspective," Schumer wrote.

"For far too long, the road toward a normalized trade and investment relationship between the U.S. and China has been a one way street -- it's time that we demand equal treatment from China."

Meanwhile on Friday, the Securities and Exchange Commission said it got a court order to freeze assets of Hong Kong traders who bought stock in Nexen before CNOOC announced its plans.

The SEC said Well Advantage Limited and other traders allegedly bought shares in Nexen based on inside information. China's CNOOC announced plans to buy Nexen on Monday for $15.1 billion.

The SEC said it moved to freeze the assets shortly after Well Advantage tried to sell all of its Nexen stock.

The friendly deal between Nexen and China National Offshore Oil Co. announced Monday needs approval from Investment Canada and the Competition Bureau to go ahead.

The NDP said Friday two of its MPs have written letters to the Chairs of the Standing Committee on Natural Resources and the Standing Committee on Industry asking them to hold public hearings on the takeover proposal.

"The size and scope of this transaction raises important questions and Canadians deserve the opportunity to seek clarity on the commitments that CNOOC is making on protecting jobs, contributing to the community and consulting affected First Nations," New Democrats wrote in their letter.

"We ask that you convene meetings of each Standing Committee at the earliest possible convenience to engage in a full and comprehensive study of the proposed acquisition of Nexen Inc by CNOOC, and of the broader policy issues raised by this case."

Saskatchewan Premier Brad Wall, speaking at the closing news conference of a premiers' meeting in Halifax Friday, said the U.S. is an "important trading partner and friend," but Canada has its own review process to go through.

He said it's up to the companies to demonstrate to Canadians their deal is of net benefit to the country.

"We in Saskatchewan, we would suggest that the process, though imperfect, has worked in the past and can certainly work again for Canadians and I'm sure that will be the case with respect to this deal," he said.

Wall staunchly argued against Anglo-Australian mining giant BHP Billiton's US$40-billion hostile bid for Saskatoon-based Potash Corp. of Saskatchewan (TSX:POT) in 2010, which was ultimately nixed by Ottawa because it did not pass the secretive net-benefit test.

The CEO of TransCanada Corp. (TSX:TRP) -- the Calgary pipeline company whose plan to ship Alberta crude to U.S. Gulf Coast refiners has met a lot of resistance -- said the Nexen deal shows the oilsands will continue to attract international interest.

"It's a very important resource and Canada is a good place to do business. It has proper regulatory and environmental oversight, rule of law. It's a good place to invest your capital in this kind of business," Russ Girling told reporters and analysts on a conference call.

"So I suspect that it will continue to attract more and more investment form foreign companies."