* Kerry calls Afghanistan-Pakistan talks productive
* Kabul thinks Islamabad wants instability
* NATO chief says Pakistan used as base by militants
Afghan President Hamid Karzai and Pakistan's army chief and a foreign ministry official held "productive" talks on Wednesday on easing tensions between the neighbouring states, U.S. Secretary of State John Kerry, who hosted the meeting, said.
Kerry cautioned, however, that any results of the Brussels talks would have to be measured in improving relations as NATO winds down its Afghanistan mission.
"We had a very extensive and ... a very productive and constructive dialogue ... But we have all agreed that results are what will tell the story, not statements at a press conference," Kerry told reporters, without disclosing any details of what was discussed.
Afghanistan has grown increasingly frustrated with Pakistan over efforts to pursue a peace process involving the Taliban, suggesting that Islamabad is intent on keeping Afghanistan unstable until foreign combat forces leave at the end of 2014.
Kerry hosted the meeting between Karzai and Pakistani army chief General Ashfaq Kayani and senior Foreign Ministry bureaucrat Jalil Jilani, with the aim of calming tensions over border disputes and the stalled peace process.
"I think that everybody here agreed today that we will continue a very specific dialogue on both the political track as well as the security track," Kerry, flanked by Karzai and Kayani, said after more than three hours of talks.
"We have a commitment to do that in the interests of Afghanistan, Pakistan and peace in the region."
After talks over lunch, Kerry, Kayani and Karzai strolled together in the sprawling garden of the residence of the U.S. ambassador to NATO on the outskirts of the Belgian capital.
Kerry told reporters at the start of the meeting that Afghanistan was in "a critical transformational period".
Karzai called it an important meeting and said he was glad Kayani and Jilani had found the time to travel to Brussels.
"Let's hope...for the best," he told reporters.
Neither Karzai nor the Pakistan officials made any comment at the end of the meeting.
The talks come a day after a gathering of NATO foreign ministers in Brussels at which alliance Secretary-General Anders Fogh Rasmussen said Pakistan must crack down on militants who use the country as a sanctuary to launch attacks in Afghanistan.
The meeting follows weeks of tension with Pakistan over their 2,600-km (1,600-mile) border and stalled peace efforts.
Afghan officials say Pakistan has a long history of supporting Afghanistan's Taliban and other insurgent factions. Pakistan in turn has accused Afghanistan of giving safe haven to militants on the Afghan side of the border.
NATO-led forces are expected to cede the lead role for security in Afghanistan this spring to Afghan soldiers, 12 years after the United States invaded Afghanistan to oust the Taliban government harbouring Osama bin Laden, the al Qaeda leader.
The White House has yet to decide how many U.S. troops will remain in Afghanistan after 2014. Much depends on progress in negotiations with Karzai on a Bilateral Security Agreement to define the future legal status of U.S. forces.