Federal party leaders squared off in their first TV debate Thursday, a mostly polite two-hour exchange of views on the economy, environment and military action overseas.

The environment segment was most focused on B.C., where Prime Minister Stephen Harper was pressed on stalled pipeline proposals to the west and east coasts as well as to the U.S. Harper said low world prices for energy make it the only sector that is under-performing, while the rest of the economy is growing.

Green Party leader Elizabeth May urged NDP leader Thomas Mulcair to oppose the twinning of the TransMountain oil pipeline from Alberta to its Burnaby terminal. Mulcair stuck to his position that he will await a federal environmental review.

Harper noted that Liberal leader Justin Trudeau has opposed capital cost allowances for liquefied natural gas investors, weakening the case for new gas pipelines. He said the government approved the Northern Gateway pipeline proposed to run to Kitimat, with more than 200 conditions because "that's how the system works."

Trudeau and Harper sparred over greenhouse gas emission efforts, with Harper noting he moved ahead on curbing coal power plant emissions three years before U.S. President Barack Obama.

Harper agreed that approval of the Keystone XL pipeline from Alberta to southern U.S. refineries will likely have to wait for Obama to leave office next year.

May said Harper is nowhere near meeting his own commitment to curb emissions, as forest fires rage and extreme weather increases all year around.

The debate moderator, Maclean's magazine columnist Paul Wells, asked Harper if he owes Canadians an apology for appointing since-suspended senators Mike Duffy, Pamela Wallin and Patrick Brazeau. Harper did not apologize, saying the provinces need a consensus to reform the senate through elections as he proposed, or to abolish it.

Mulcair and Trudeau oppose the current bombing missions against Islamic State terrorist targets in Syria and Iraq. Mulcair said he would support military action sanctioned by the United Nations or under Canada's NATO commitments.

Harper said the deployment of six fighter jets with special forces support is what Canada's allies want to do, and Islamic State is a "nerve centre" for a global movement that has targeted Canada specifically.