Mr Johnston acknowledged that the value of the euro had strengthened against the pound recently but he added that "the movement is not as substantial as we would like".
Speaking after a meeting with representatives from Iron and Steel Trade Confederation, Corus Executive Director Allan Johnston said that people affected would be the first to know.
The meeting took place amid growing concern over more redundancies and possible closures.
Mr Johnston acknowledged that the value of the euro had strengthened against the pound recently but he added that "the movement is not as substantial as we would like".
On the issue of compulsory redundancies, Michael Leahy, General Secretary of the ISTC, said that he was not aware that there were problems in Wales for the moment.
He said that his understanding was that the major Welsh plants were shedding jobs through voluntary redundancies.
But there is continuing speculation that the company may shut its giant Llanwern plant near Newport which employs 3,000 people.
The meeting in London on Monday was scheduled to discuss last year's redundancies - most of which have yet to take effect.
Compulsory redundancies
Corus is already cutting 4,500 jobs across the UK and at some plants a lack of volunteers means the company is imposing compulsory redundancies.
This is being vigorously resisted by the unions and a ballot on strike action has already been called at the Scunthorpe plant.
The possibility of similar moves in Wales is not being ruled out by the unions.
"We have made our position quite clear to Corus and have asked them to withdraw compulsory redundancies."
The loss of Llanwern would be a devastating blow for the Newport area.
It is estimated Llanwern accounts for around 10,000 jobs - both at the site and in dependent businesses.
'Weak UK demand'
Last September news that Corus would spend £35m relining one of the plant's blast furnaces appeared to indicate that most of the plant's jobs were safe.
But in December Corus said further radical action was needed because of weak UK demand, oversupply throughout Europe and the strong pound, which was hindering exports.
Also in December Welshman John Bryant and his fellow chief executive Fokko van Duyne resigned.
Analysts said the departure of Mr Bryant, who had been seen as a strong supporter of the steel industry in Wales, probably spelled the end for Llanwern.
Before Christmas Wales's First Minister Rhodri Morgan joined Welsh Secretary Paul Murphy at talks with Corus's chief executive at the Department of Trade and Industry in London.