Pressed by North American allies on an array of politically fraught issues, U.S. President Barack Obama on Wednesday vowed to press ahead with stalled efforts to expand trade agreements for the Americas into Asia and overhaul fractured U.S. immigration laws. But Obama made no promises about his long-anticipated decision on the Keystone XL pipeline.

While Obama acknowledged that the U.S. review of the pipeline project has been "extensive," he defended the process, saying "these are how we make these decisions about something that could potentially have significant impact on America's national economy and our national interests."

"There is a process that has been gone through and I know it's been extensive and at times, I'm sure Stephen feels, a little too laborious,"Obama said in a joint news conference with Prime Minister Stephen Harper at the Three Amigos summit 

 "But these are how we make these decisions about something that could have significant impact on America's national economy and our national interest. So, the State Department has gone through it's review. There's now a comment period in which other agencies weigh in. That will be evaluated by Secretary of State Kerry and we'll make a decision at that point,"

Harper added: "In terms of climate change, I think the State Department report was pretty definitive on that particular issue."

Stephen Harper in Mexico

Prime Minister Stephen Harper met with Mexican president Enrique Pena Nieto one-on-one Tuesday in Mexico City. On Wednesday he met with officials from Keystone XL pipeline owners TransCanada. (Sean Kilpatrick/Canadian Press)

A final decision on Keystone isn't expected until this summer, at the earliest, meaning the verdict could potentially come in the run-up to November's midterm elections, in which energy issues are likely to be a factor in some key races. 

For Canada, a key source of frustration with the U.S. has been the Obama administration's long and drawn out review of the Keystone XL pipeline, which would carry oil from tar sands in western Canada to Nebraska, where existing pipelines would then carry the crude to refineries on the Texas Gulf Coast. Canada has been pushing the U.S. for years to approve the pipeline, but environmental groups oppose it, and Obama has said he won't approve it if it increases greenhouse gas emissions.

A Nebraska judge on Wednesday struck down a law that allowed the pipeline to proceed through the state, a victory for opponents who have tried to block the project.

Closing a day of talks with the leaders of Mexico and Canada, Obama said the North American partners must maintain their "competitive advantage" on trade, in part by expanding into the fast-growing Asia-Pacific region. And he downplayed opposition to the Trans-Pacific Partnership agreement from members of his own Democratic Party on Capitol Hill.

"We'll get this passed if it's a good agreement," Obama declared during a joint news conference with Mexican President Enrique Pena Nieto and Canadian Prime Minister Stephen Harper.

The North America Leaders' Summit — often referred to as the "Three Amigos" meeting — coincided with the 20th year of the North American Free Trade Agreement among the three countries, a deal that has vastly expanded cross-border commerce in the region but which remains a contentious issue in the United States over its impact on jobs and on environmental protections.

Trade experts say the agreement is due for an upgrade to take into account the current globalized environment and to address issues not touched in the original pact. But rather than reopen NAFTA, the three countries are instead relying on negotiations underway to complete the TPP, which is a trade bloc of 12 countries in the Americas, Asia and the Pacific.

Pena Nieto and Harper echoed Obama's support for expanding North American trade into Asia and the Pacific, with the Canadian leader saying he was "focused on bringing those negotiations to a successful conclusion."

Despite the widespread agreement on trade, there were some sources of tension between the North American partners on immigration and the Keystone XL pipeline, both sensitive political issues in the United States.

With files from Reuters, The Canadian Press