THE HAGUE, Netherlands — Nuclear terrorism is officially the main topic for world leaders at a two-day summit in the Netherlands starting Monday. In practice, the Ukraine crisis will likely overshadow those talks.

The Nuclear Security Summit in the Hague will form the backdrop for an emergency meeting of Group of Seven leaders on Russia’s annexation of Crimea. It’s a confrontation between Russia and the West reminiscent of the Cold War.

Russian President Vladimir Putin is not attending, instead sending Foreign Minister Sergey Lavrov, who is expected to hold talks with U.S. Secretary of State John Kerry.

Dutch Prime Minister Mark Rutte said the event’s timing means world leaders can discuss Ukraine and Russia face to face.

“I think these multilateral summits are an excellent opportunity for world leaders to discuss bilaterally and also amongst smaller groups of countries various issues which are high on their minds,” Rutte said.

But experts say frantic diplomacy focused on Ukraine shouldn’t divert from the goal of better security of nuclear material.

“International attention can turn in a moment,” said Deepti Choubey, a senior director at the non-government Nuclear Threat Initiative. “The attentions of terrorists do not.”

Delegations from 53 countries, including China and Japan, have started to arrive in the Hague. They will meet to negotiate on reducing and securing supplies, and keeping them out of terrorists’ hands. The G-7 includes the U.S., Britain, Germany, Japan, France, Italy and Canada.

Notable absentees from the summit are North Korea and Iran, which were excluded by mutual consent.