Barack Obama has said the United States will levy new sanctions against Russian individuals and companies in retaliation for Moscow's alleged provocations in Ukraine.

Obama said the targets of the sanctions would include hi-tech exports to Russia's defence industry, as he seeks to increase pressure on Vladimir Putin, the Russian president.

The full list of targets, which will be announced by officials in Washington on Monday, is also expected to include wealthy individuals close to Putin.

"The goal here is not to go after Mr Putin personally," Obama said. "The goal is to change his calculus with respect to how the current actions that he's engaging in could have an adverse impact on the Russian economy over the long haul."

Obama announced the sanctions during a news conference in the Philippines, his final stop on a four-country Asia tour.

White House officials say they decided last week to impose the new sanctions after deciding that Russia had not lived up to its commitments under a fragile diplomatic accord aimed at easing the crisis in Ukraine. But the US held off on implementing the sanctions in order to co-ordinate its actions with the EU, which could also announce new penalties as early as Monday.

The failed diplomatic accord reached in Geneva called on the Kremlin to use its influence to get pro-Russian insurgents to leave the government buildings they have occupied in eastern Ukraine. But those forces have not left the buildings and have also stepped up their provocations, including capturing European military observers.

Even as he announced the new sanctions package, Obama acknowledged that "we don't yet know whether it's going to work".

In eastern Ukraine, the situation remained jittery after pro-Russia militants holding the European observers paraded them in front of the world's media on Sunday as "prisoners of war".

AFP reporters said gunmen were increasingly aggressive at checkpoints and occupied buildings in the flashpoint eastern Ukraine town of Slavyansk, one of a handful under control of pro-Kremlin rebels.

Meanwhile heavily armed gunmen on Monday seized control of the town hall in nearby Kostyantynivka, according to an AFP reporter on the scene, the latest in a string of about a dozen towns and cities in south-eastern Ukraine to fall under the control of pro-Russian insurgents over the past few weeks.

There are fears of a full-scale invasion, with tens of thousands of Russian troops massed on the border and Ukraine's prime minister warning of efforts to start a "third world war".

As the west stepped up the rhetoric ahead of the sanctions announcement, the French foreign minister Laurent Fabius warned the Ukraine crisis could yet have "incalculable consequences".

On Monday, EU officials are to meet to thrash out a new set of sanctions likely to include asset freezes and travel bans.

Diplomats in Brussels have already agreed in principle to add 15 people to a blacklist of 55 Russians and Ukrainians.

The G7 has vowed to act "swiftly" to raise the pressure on Russia, where the economy is already contracting.

Speaking on his tour of Asia, Obama said it was vital to avoid "falling into the trap of interpreting this as the US is trying to pull Ukraine out of Russia's orbit, circa 1950. Because that's not what this is about."

The west has already imposed visa and travel restrictions on key members of Putin's inner circle and slapped sanctions on a top bank.

But some fear that undermining Russia's economy could tip the world back into recession, just as it begins to recover from the effects of the eurozone debt crisis.

Reflecting these jitters, stocks in Japan closed nearly 1% lower and the yen – seen as a safe haven against uncertainty – gained ground against the dollar.

However, the former Russian tycoon Mikhail Khodorkovsky said the sanctions would have "no short-term effect" on the Russian economy and would take three or four years to really hurt Putin.

In Slavyansk, intense negotiations continued to secure the release of a team of the seven monitors from the Organisation for Security and Cooperation in Europe (OSCE) captured by pro-Russian rebels. Germany, in command of the mission, said parading the observers in front of the world's media was "repugnant".

Speaking in Slavyansk town hall and watched by four armed guards and the self-styled mayor, Vyacheslav Ponomaryov, the group's leader, German officer Axel Schneider, told reporters they were in good health.

One of the group, a Swedish monitor, was released late Sunday on medical grounds as he suffers from diabetes and a team of OSCE negotiators was in talks to free the rest.

But the rebels have described them as "Nato spies" and "prisoners of war" and refused to release them except as part of an exchange for their own detained militants.

OSCE chairman and Swiss president Didier Burkhalter said his organisation was working "at all levels" to secure their release.

The German foreign minister, Frank-Walter Steinmeier, said the presentation of the detained OSCE men in front of the press was "a violation of all negotiating rules and norms that prevail in tense situations like this one".

The monitors were in Ukraine to assess progress towards implementing the deal struck in Geneva on 17 April.

The rebels are also holding three Ukrainian soldiers captured near Slavyansk. Russian television showed the men blindfolded, bloodied and stripped to their underwear.

The fate of four Ukrainians travelling with the OSCE group is also unknown, as they did not appear at the news conference with the eight Europeans – four Germans, a Dane, a Czech, a Pole and the Swede who was later released.