* Kerry says "year of transition" for Afghanistan
* Meeting comes after NATO ministers discuss troop withdrawal
* Afghanistan, Pakistan at odds over peace process
U.S. Secretary of State John Kerry will host talks between Afghan President Hamid Karzai and senior Pakistan officials in Brussels on Wednesday, officials said, with the aim of calming tension over border disputes and a flagging peace process.
The meeting is part of a series of on-off discussions between Afghanistan and Pakistan at the behest of the United States, a senior State Department official said on Monday, confirming that Kerry had offered to host the gathering.
Afghanistan has grown increasingly frustrated with Pakistan over efforts to pursue a peace process involving the Taliban, suggesting that Islamabad is intent on keep Afghanistan unstable until after foreign combat forces have left at the end of 2014.
Kerry said the meeting would discuss the handover of security responsibility to Afghan forces this year, a move intended to allow for the end of NATO-led combat operations.
"This is the year of transition. This is the critical year in Afghanistan," he told U.S. diplomats in Brussels.
"We are going to have a trilateral and try to talk about how we can advance this process in the simplest, most cooperative and most cogent way, so that we wind up with both Pakistan's and Afghanistan's interests being satisfied, but, most importantly, with a stable and peaceful Afghanistan which is worth the expenditure and the treasure and effort of these last years."
U.S. officials are hopeful that Kerry, who has a good relationship with Karzai, can bring the parties back to the negotiating table and make constructive progress on an issue that has long-term security implications for Washington.
An Afghan spokesman said earlier that Karzai would travel to Brussels for the talks, which follow weeks of tension with Pakistan over their 2,600 km (1,600 mile) border and stalled peace efforts.
"Our message to Pakistan is enough is enough," Karzai's spokesman, Aimal Faizi, said in Kabul. "This time we will tell Pakistan that our people's patience is running out and we can't wait for Pakistan to deliver on Afghan peace promises."
Last month, Afghanistan's deputy foreign minister called Pakistan "complacent" when it came to the nascent peace efforts and said it was ready to work on reconciliation with Taliban groups without Pakistan's help if necessary.
A public slanging match ensued, with the Pakistani Foreign Ministry accusing Karzai of being an "impediment" to the peace process. [ID L3N0CC17D]
Although there have been several meetings in Western capitals over the past few months in which representatives of the Taliban have met Afghan peace negotiators, there have been no signs of a breakthrough.
Kabul accuses Pakistan of harbouring the Taliban leadership in the city of Quetta and using militants as proxies to counter the influence of India in Afghanistan.
Publicly, the Taliban say they will not engage in peace talks with the Karzai government.
As well as Karzai and Kerry, the meeting will include Afghanistan's defence minister, Bismillah Khan Mohammadi, Pakistan's army chief, General Ashfaq Kayani, and Pakistan's foreign secretary, Jalil Jilani, the U.S. official said.
The talks will take place the day after a meeting on Tuesday of NATO foreign ministers in Brussels, which Kerry will attend. That meeting will discuss the process of transition and the shift in the role of foreign troops from combat to training, advising and assisting Afghan forces.
NATO ministers agreed in February that they would think in terms of no more than 8,000-12,000 NATO troops remaining in the country after 2014 compared with about 100,000 now.
The United States had yet to decide how many troops it will keep in the country and U.S. officials say much will depend on negotiations between the United States and Afghanistan on the legal status of those troops.
Karzai's spokesman said hopes for a breakthrough at Wednesday's talks were slim. "Discussions have been warm and friendly in the past but Pakistan unfortunately did not take any practical steps," Faizi said.
Earlier this month there was outrage in Afghanistan over the building of a Pakistani military outpost in a border area of Nangarhar province which the Afghan Defence Ministry says was inside Afghan territory.