UN envoy Ibrahim Gambari has described as "extremely disturbing" new arrests in Burma, calling on the ruling junta to stop detaining democracy activists.
Mr Gambari aims to enlist support from Burma's neighbours
EU foreign ministers later agreed to broaden existing sanctions against the junta, to target Burma's key timber, metals and gem exports.
Several prominent Burmese student leaders were arrested over the weekend.
Mr Gambari said the detentions ran "counter to the spirit of mutual engagement" between the UN and Burma.
He was speaking in Thailand as EU foreign ministers gathered in Luxembourg to discuss tougher sanctions against the junta.
The UN envoy met Thai Foreign Minister Nitya Pibulsonggram at the start of a six-nation Asian tour aimed at enlisting support from Burma's neighbours for tougher action against the Rangoon regime.
Mr Gambari is due to visit Malaysia, Indonesia, Japan, India and China, before returning to Burma for a follow-up visit.
Earlier this month, he met senior junta officials as well as detained opposition leader Aung San Suu Kyi.
Burma's generals already face an EU travel ban and a freeze on assets.
The generals now face fresh embargoes on some of their more lucrative exports - including jade, rubies and teak.
However, European sanctions have had practically no impact on Burma, as more than 90% of the country's trade is with its Asian neighbours, BBC European affairs correspondent Oana Lungescu says.
The new measures will not prevent the French energy giant Total from doing business with the junta, though diplomats say that may come later.
Ahead of the meeting in Luxembourg, UK Prime Minister Gordon Brown led calls for tougher sanctions.
"We cannot forget the images on our television screens of monks and ordinary citizens in Burma protesting, nor the death and human rights abuses we know are still taking place," he said.
Burma's security forces at the weekend rounded up three of the remaining leaders from last month's demonstrations, according to human rights group Amnesty International.
Monks led last month's violently suppressed street protests
Last week, the UN Security Council condemned the generals' crackdown on the protests.
The regime said it "deeply regretted" the statement.
Burma says it arrested about 100 monks in recent weeks and that only 10 people died during its crackdown on protests, but correspondents say the true figures are probably much higher.