OTTAWA - Prime Minister Stephen Harper is raising the possibility of a "significantly modified" takeover deal between a Chinese oil company and Nexen getting the thumbs up from the Conservative government.

"Our position has been to be generally welcoming of foreign investment," Harper said Thursday. "We've approved many transactions, we have significantly modified some and we have blocked some transactions."

The three possibilities are all contained within the Investment Canada Act that guides reviews of major foreign investments, but it's the first time Harper has mentioned modifying the $15-billion deal in any discussion of the takeover.

Meanwhile, the NDP is officially calling on the federal government to reject the merger because of state-owned CNOOC's record on labour, environmental and human rights issues, as well as national security concerns.

"CNOOC operations are tied directly to the Chinese government," said natural resources critic Peter Julian.

He also accused the government of not being clear with Canadians about how it will reach a decision.

"They've completely botched this file," said Julian.

None of that seemed to make an impression on the prime minister.

"The NDP is an ideologically socialist party that's opposed to all investment," said Harper.

Still, Harper acknowledged the decision on the takeover is tough.

"This particular transaction raises a range of difficult policy questions, difficult and forward-looking issues, and those things will all be taken into account under the act in assessing the net benefit of this investment to this country before we take a decision," he said.

Harper is also facing American pressure to reject the Nexen takeover, QMI Agency has learned.

In an exclusive report, QMI's French-language network, TVA, says Washington and U.S. business leaders are pressing the Conservatives to block CNOOC from establishing itself in the North American market.

Harper says the decision is Ottawa's to make.

"I don't think it's a surprise for me to tell you that the government of Canada will take its own decision irrespective of what the government of the United States does," he said. "We don't, obviously, follow their judgement in these matters."