SEOUL, South Korea - Prime Minister Stephen Harper is homeward bound, after concluding his three-nation Asian tour at the nuclear security summit in Seoul, South Korea.

The prime minister left the two-day summit having pledged Canada's commitment to spend $367 million over five years, through a federal program managed by the department of Foreign Affairs, on international efforts to dismantle, destroy or secure nuclear and chemical weapons. Since its creation in 2002, the fund has seen $820 million spent on activities including radiation detection and the decommissioning of nuclear facilities and materials across the former Soviet Union.

Harper also announced an agreement to speed the pace of sending spent highly enriched uranium from Canada's Chalk River facility to the U.S., and pledged to add another $5 million to the $17 million Canada has contributed to the International Atomic Energy Agency Nuclear Security Fund.

"Canada's committed to securing nuclear material from around the world," Harper said, singling out nuclear terrorism as "one of the most challenging threats to nuclear security today."

But as he concluded his attendance at the 53-nation summit Tuesday, Harper made it clear why trade has topped his agenda throughout this latest Asian swing.

"I don't think it's any secret, if you talk to most leaders today, the economic circumstances of the global economy and their own economies are very much centre and very much top of mind," Harper said, explaining that issue overshadowed security in his bilateral meetings with Italy, India, Spain and the European Union during his time in Seoul.

"We're working hard here to work with others and increase jobs, growth and long-term prosperity. Ultimately our view as a trading nation, in a global economy, all interests are linked," he said.

Harper was forced to address one tangle among all those links, however, on the subject of a Canadian company sticking with a deal to sell medical isotopes produced from weapons-grade uranium.

While the rest of the world moves away from the practice, Ottawa-based Nordion Inc. is just two years into a 10-year deal with Russia.

When asked how his government can reconcile its pledge to both reduce the use of the weapons-grade uranium with its support if the Nordion contract, Harper answered carefully.

"We are committed over time to eliminating that practice, to moving away from that," he said. "We have said we would not do it instantly, but we are making investments specifically on alternatives to that kind of isotope production in Canada. We'll continue to do so and we will fulfill our commitments."

Trade Tops Agenda

Trade and investment topped the agenda throughout Harper's 6-day tour, starting on day one in Thailand, where he and Thai Prime Minister Yingluck Shinawatra announced their two countries would begin setting the stage for free trade talks.

Free trade talks also grabbed headlines when Harper arrived in Japan, as the prime minister hailed the opening of talks with Tokyo as an "historic opportunity" for Canadian businesses to gain access the world's third-largest economy.

And before leaving Seoul for the long journey back to Ottawa, Harper made it clear he's aware of the world's pressing security concerns too.

"I think there is virtually a universal assessment, that Canada would share, that the recent actions by North Korea by launching a satellite are a contravention of UN resolutions," he told reporters.

"We would obviously attempt to encourage them, dissuade them from any such action. As you known, unfortunately, this regime seems to delight in irritating the international community."

Underscoring the prime minister's comments in a message posted to the social networking website Twitter, Foreign Affairs Minister John Baird wrote: "Recent North Korean sabre-rattling on potential missile launch is unacceptable."

The missive was sent, Baird tweeted, from the Demilitarized Zone separating North and South Korea just 50 kilometres from the summit site.

The minister and a handful of MPs accompanying the prime minister on his Asian tour made the unannounced visit as Harper wrapped up his appearance at the summit.

The prime minister and his entourage could be back in Ottawa as early as Tuesday night, giving them a day to recover before the federal budget is tabled Thursday.