Fifteen of NATO's 29 members have laid out plans to meet the alliance's defense spending goal by 2024, overall increasing spending by $46 billion, Secretary-General Jens Stoltenberg said.

"This is substantial progress, and a good start," he said on February 13 ahead of a NATO defense ministers meeting in Brussels this week. "After years of decline, since 2014 we have seen three years of increasing defense spending across European allies and Canada."

Stoltenberg's comments come as U.S. Defense Secretary Jim Mattis is expected to pressure U.S. allies in Europe to increase military spending to levels targeted by NATO, fulfilling a key commitment sought by U.S. President Donald Trump.

NATO has set a goal of each member spending 2 percent of their gross domestic product on defense, but until recently only a few of NATO's 29 members met that target.

Stoltenberg said that in 2014, only three allies met the goal, but the number has increased to eight this year. The increase in the last four years has added $19 billion to spending on weapons and equipment for the alliance, he said.

An additional seven NATO members have laid out plans to meet the goal by 2024, he said.

"This should lead to significant improvements in our forces and their readiness," he said, "but we still have a long way to go."

According to NATO, Britain, Greece, Romania, Estonia, Latvia, and Lithuania have met the 2 percent goal, while France and Turkey are among the countries set to reach it soon.

France is envisioning a dramatic increase in spending of more than one-third by 2025. But important NATO members remain far short of the goal.

A large projected increase in military spending by Germany will not be enough to take Berlin up to 2 percent by 2024. Spain, Belgium, and Italy also have said they will not meet the target by 2024.

Trump has since taking office last year pushed hard for greater burden-sharing by Europe and Canada. The United States for decades has borne the biggest defense budgets and accounted for the lion's share of NATO spending.

U.S. Ambassador to NATO Kay Bailey Hutchison said on February 13 that Washington will continue to push for higher spending by NATO partners.

"We talk 2 percent, but that's because in the overall, that is what we need to have the capabilities and the ability to withstand any kind of threat," Hutchinson said.

Mattis is also expected to take a tough stance at the NATO gathering, said Katie Wheelbarger, principal U.S. deputy assistant secretary of defense for international security affairs.

"He will address those who don't have national plans to meet 2 percent and suggest they really need to develop those plans," she told reporters on February 13.

U.S. officials say Trump set an example this week by proposing a $1.7 billion increase in U.S. military spending in Europe in his 2019 budget.

With reporting by dpa and Reuters