Mr Trump on Wednesday urged Nato allies to commit 4% of their annual output (GDP) to military spending.
He also singled out Germany for criticism over its defence spending.
Nato leaders are hoping for a more conciliatory tone on Thursday before Mr Trump leaves for the UK.
In early-morning tweets from Brussels, the US president resumed his criticism.
He earlier accused Germany of being held prisoner to imported Russian energy, claiming it is “totally controlled” by Moscow.
EU figures suggest Russia is responsible for between 50% and 75% of Germany’s gas imports, but gas makes up less than 20% of Germany’s energy mix for power production.
President Trump’s next stop after Belgium is the UK, where he will arrive early afternoon to begin a two-day working visit.
What’s the plan for Afghanistan?
Afghanistan’s President Ashraf Ghani will be present for the second day of talks, and Nato’s Secretary-General Jens Stoltenberg hopes the bloc will agree to fund Afghan security forces until 2024.
Britain’s Prime Minister Theresa May has already confirmed that the UK will send 440 more troops to serve in non-combat roles in Afghanistan.
The US also committed an extra 3,000 troops to support Afghan forces in September 2017 – bringing its total to 15,000.
US officials have said Washington is planning a strategic review, a year after Mr Trump agreed to remain involved in the 17-year conflict.
The US-led invasion drove the hardline Taliban from power in 2001, as part of a crackdown on Islamist militants after the 9/11 attacks in the US.
Thousands of Nato troops were deployed and a long, bloody conflict followed as the ousted militants fought back.
In 2014, Nato formally ended the combat mission, handing over to Afghan forces, whom it had trained.
Since then, the Taliban have made substantial territorial gains across the country. A BBC study from January found the group is openly active in 70% of Afghanistan, and controls or threatens more territory than before the foreign troops left.
On Thursday, officials in Afghanistan said the Taliban had launched an attack on a number of army posts in the northern province of Kunduz, killing at least 30 soldiers.
It comes a day after dozens of Islamic religious scholars met in Saudi Arabia to discuss Afghanistan’s war and called on the Taliban to renounce violence.
Is Nato still expanding?
Nato leaders will discuss the alliance’s ties with Georgia and Ukraine. Both countries want to join Nato, but their bids are not expected to make progress.
That is because Russia has engaged in territorial incursions into both states, and Nato rules prevent countries with a land conflict from joining.
By contrast, a ceremony will be held for Macedonia after it was invited to begin accession talks on Wednesday.
Where is Trump going next?
When the Brussels summit winds up, Mr Trump will arrive in the UK on Thursday for a two-day working visit.
Mr Trump is the 12th sitting US president to make such a trip, and will spend time with the Queen and prime minister before flying to Scotland to spend the weekend at his golf resort.
Thousands of people are expected to protest against his visit across the UK.
Mr Trump is due to hold his first summit with Vladimir Putin, in Helsinki, on 16 July.
He shocked some by quipping that the Nato gathering might prove tougher than next Monday’s summit with Mr Putin.