The Conservatives called on Prime Minister Justin Trudeau to 鈥渆xplore the option of Canada joining the (U.S.) anti-ballistic missile program鈥� Tuesday and Defence Minister Harjit Sajjan suggested he鈥檚 not ruling it out.

that Canada must consider joining the program that aims to shoot down warheads at high altitudes as the 鈥淣orth Korean threat continues to intensify.鈥�

Sajjan later told reporters that 鈥渨hen we look at what BMD (ballistic missile defence) is, what the threats are, we need to actually spend more time to making sure that we do get this right.鈥�

The Liberals under Paul Martin declined to join the American program in 2005, angering the U.S. so much that then-secretary of state Condoleezza Rice cancelled a visit to Ottawa.

The Conservatives under Stephen Harper stuck with the decision.

Former CSIS director Richard Fadden, who served as deputy minister of defence from 2013 to 2015, told CTV鈥檚 Power Play that he believes 鈥渋f we鈥檙e going to take our security seriously 鈥� we have to participate.鈥�

鈥淚t doesn鈥檛 necessarily mean that we have to have (U.S. Air Force) anti-missile batteries in Canada,鈥� he added. 鈥淚t could mean we put specialized radars on our ships. It could mean add people to NORAD. It could mean we participate in research and development.鈥�

Fadden said that he believes the threat from missiles is higher than it was three or four years ago, making a discussion of the program difficult to ignore.

鈥淚t鈥檚 a very contentious issue politically,鈥� Fadden added. 鈥淎 lot of Canadians recoil in horror at the very thought.鈥�

Last month, Lt.-Gen. Pierre St-Amand testified that, under current policy, the U.S. would not shoot down a ballistic missile that was headed for Canada.

NDP MP Rachel Blaney told Power Play that there are 鈥渟erious concerns鈥� about North Korea and that Canada should be taking action by signing on to the United Nations Treaty Banning Nuclear Weapons, which has been agreed to by more than 120 countries but not Canada, the U.S., France or the United Kingdom.

The Union of Concerned Scientists has long questioned the cost and effectiveness of U.S. missile defence program, which has already cost an estimated $100 billion.

With files from The Canadian Press