The president of Myanmar pledged Monday to release all political prisoners in his country by the end of the year as he visited Britain for the first time.

Thein Sein, a former general who retains close ties to the military, has introduced democratic changes after decades of authoritarian rule that had led to international isolation.

He said Monday that thousands of prisoners already have been released from Myanmar's jails as the country shifts away from military rule, and that a committee is working through the cases of those still behind bars.

"I guarantee to you that by the end of this year, there will be no prisoners of conscience in Myanmar," he told an audience at Chatham House in London, shortly after meeting with British Prime Minister David Cameron.

Cameron — who had said that Britain welcomed the president's reforms and looked forward to free and fair elections in 2015 — also raised concerns during the meeting about attacks on minority Rohingya Muslims in the predominantly Buddhist nation. Attacks on the Rohingya have killed hundreds in the past year and uprooted about 140,000 people.

Observers fear the violence presents a threat to Myanmar's transition to democracy because it could encourage security forces to re-assert control.

"We are also very keen to see greater action in terms of promoting human rights and dealing with regional conflicts," Cameron said. "We are particularly concerned about what has happened in Rakhine province and the Rohingya Muslims."

In his remarks at Chatham House, Thein Sein said recent violence in Myanmar has "rightly concerned the world."

"I promise you that we will take a zero-tolerance approach to any renewed violence and against those who fuel ethnic hatreds," he added.

Thein Sein's visit was met by some protests in London outside of Parliament.

Ricken Patel, executive director at human rights campaign group Avaaz, said the attacks on the Rohingya "should be ringing alarm bells" everywhere.

"Cameron has a responsibility to use all his diplomatic leverage to get real, concrete measures from President Thein Sein to protect these groups," Patel said.