Wed Oct 3, 1:22 PM ET

YANGON - Myanmar's military regime kept up the pressure on its people Wednesday after last week's bloody crackdown on protesters as the European Union agreed in principle to punish the junta with sanctions.

Troops who last week killed at least 13 and arrested over 1,000 people to suppress the largest pro-democracy protests in nearly 20 years have continued overnight arrests and mounted patrols to strike terror into the population.

"You must stay inside. Don't come out," soldiers said through blaring loudspeakers as they drove around Myanmar's biggest city Yangon. "We have photographs of the people we're looking for. We will arrest them."

In one pre-dawn raid, the regime detained a local UN staff member, her husband and two relatives, said UN resident coordinator Charles Petrie.

Last week's bloody crackdown -- details and images of which have trickled out on web postings and video clips despite a media clampdown -- has drawn international outrage, and governments have started to take action.

The European Union has agreed in principle to toughen sanctions against the regime in Myanmar, the EU's Portuguese presidency said.

The EU already has broad sanctions in place on Myanmar's leadership but has since been preparing new measures while stressing the importance of pressure on neighbouring countries and in particular China and India.

Japan also said it was considering cutting aid to Myanmar following the shooting dead of its video journalist Kenji Nagai.

UN envoy Ibrahim Gambari was Wednesday readying a report on his talks with Myanmar's ruling generals and democracy leader Aung San Suu Kyi this week, amid global concern over the violent crackdown on anti-government protests.

Local and UN officials say that at least 1,000 disrobed monks and activists are being held inside a windowless warehouse at a Yangon campus, and there are unconfirmed reports about more mass arrests.

Many residents have said monks who used to live in neighbourhood monasteries were missing -- possibly injured, arrested or forced back to their villages.

"People feel angry, although they know they cannot fight against the military," one 45-year-old man said. "But they want the monks to come back."

In Yangon, troops have reduced their presence on the streets but soldiers with sniffer dogs patrolled bus stops and markets.

Military trucks were still parked near Yangon landmarks which were flashpoints in the demonstrations, including the city hall and Sule Pagoda which is near the spot where a Japanese journalist was killed.

As security forces in Myanmar's main city Yangon maintained a tight grip, UN special envoy Gambari was in Singapore, from where he was to fly back to New York to brief UN chief Ban Ki-moon, probably on Thursday, and the UN Security Council.

"I have to brief the secretary general of the United Nations who sent me and also the Security Council. Before that I can't say much," Gambari told reporters as he left his hotel late Wednesday, after refusing comment all day.

Over four days, Gambari criss-crossed Myanmar for talks with junta leaders in their remote capital Naypyidaw and, on two separate occasions, with Nobel Peace Prize winner Aung San Suu Kyi, who is detained at her Yangon home.

But a former mediator who helped broker landmark contacts between the junta and the opposition warned the crisis was far from over and that the generals had shown no sign they are ready to compromise.

Leon de Riedmatten, who served as an informal mediator for the United Nations, said he hoped something positive would come from Gambari's visit but warned that "we must also be very realistic about the situation."

Myanmar's generals have "never negotiated anything, they always impose their position and their will, and I do not believe that has changed today," he said.

In a sign that at least some officers had qualms about firing on their own people, one mid-ranking soldier has fled to neighbouring Thailand in the first known defection since the crackdown.

"As a Buddhist myself, when I heard that monks had been shot dead on the streets and that other people had been shot dead, I felt very upset," he said in a video interview, a copy of which was made available to AFP.

"As a Buddhist, I did not want to see such killing," said the man, who asked not to be identified.

The protests first erupted in mid-August after a massive hike in the price of fuel, but escalated two weeks ago when the revered monks emerged to lead the movement, drawing up to 100,000 people onto the streets.