The cautious step-by-step ratcheting up of economic pressure on Russia over its intervention in Ukraine continued on Wednesday, March 12, when David Cameron said the UK was backing the imposition of a travel ban on senior Russian politicians, The Guardian reports.

Cameron made his announcement the day after senior European Union, American, Japanese, and Swiss officials met in London to look at asset freezes and travel bans on senior Russian officials, including military officials.

His announcement is confined to a travel ban, omitting any reference to an asset freeze, or to the military.

Cameron is targeting sponsors of legislation in the Russian Duma that would accept Crimea as part of Russian territory in the wake of a secession referendum due this Sunday.

It would not extend to MPs that simply supported the legislation. The enabling law had been due to rushed through the Duma this week, but has now been delayed until Monday.

The travel ban would apply to visits to the EU, but Britain would like to widen the net to other countries rapidly. British officials gave no reason why an asset freeze had been deferred, but the aim is to graduate the pressure in the hope that it will persuade the Russian president, Vladmir Putin, to de-escalate the crisis by agreeing to set up a diplomatic "contact group" to discuss Ukraine.

The west has set out a three-phase process starting with visa restrictions, moving to a travel ban and asset freezes, and then culminating in wider economic sanctions.

"I want confirm we would be pushing within the EU for those travel bans to include some prominent Russian MPs. We have put that on the table. I am not naming names today," Cameron said, according to The Guardian.

He said the ban would be imposed on MPs that "have been pushing for the unacceptable steps that have been taken

He denied that the west's response continued to be inadequate, saying the EU meeting last week surprised everyone who was expecting the U.S. to take a series of steps and for the EU to fall short.

"Actually Britain played an important role in bringing everyone together like the Poles that want to see strong action, and then to forge a consensus. The three-phase process sets Europe on a pathway; and it would make subsequent meetings – and there will be subsequent meetings certainly of foreign ministers and possibly of heads of government – [and there will be] a point at which subsequent phases can be triggered as necessary."