JEREZ, Spain (Reuters) - Mercedes called a halt to their first day of 2013 Formula One testing on Tuesday after Nico Rosberg stopped on track with an electrical problem and flames flaring briefly around the back of his car.

Mercedes Formula One racing driver Nico Rosberg of Germany walks at the Jerez racetrack in southern Spain February 4, 2013. REUTERS/Marcelo del Pozo

The team said the gremlins would be fixed before Lewis Hamilton, the 2008 world champion, makes his test debut with them on Wednesday.

"After identifying the cause of the problem, we will not be running again today whilst parts are modified ready for tomorrow," Mercedes said on their Twitter feed.

"The part which requires modification is the wiring loom where the electrical fault originated this morning."

Mercedes were fifth overall last season and have struggled to impose themselves since taking over the world championship-winning Brawn GP team in 2009.

Rosberg completed 14 laps at the Jerez circuit in the F1 W04 car that he and Briton Hamilton, who has moved from McLaren, unveiled to the world at the Spanish circuit on Monday.

Despite the lack of running, the German said his first impressions were "very good, very positive".

"I can guarantee you it will be a big step forward, but of course everybody else is also moving forward and we're hoping that we've closed the gap," he told reporters outside the Mercedes hospitality.

"Last year it was nearly more than a second...and we have to get closer for sure and we will, we will get closer," added the son of 1982 champion Keke.

"I think they've given us a good base now and it's up to us make the most of it here, find all the problems as we've done today and also improve it and feed back to the factory what are the main things that are still holding us back, what are the main things they need to keep working on."

Rosberg, starting his fourth season with the British-based team, said it was too early to say whether the car was quick enough but it was enjoyable to drive and the balance was good.

Rosberg, who won in China last year, said he was sure the team could be competitive and dismissed a suggestion that the focus might quickly turn to 2014, when the regulations undergo a major change.

"We are fully concentrated on 2013 because we as Mercedes need to make it happen also this year. So it's full speed ahead for 2013," he said.

"Improvement can also be in being more consistent through the year. Not like last year where we started OK and then completely fell off.

"Improvement would also be a very OK car through the whole season with a win or two maybe. What we need is progress, we need to move forward and go through the season thinking now we are on the right track to become the best."

The 2013 season starts with the Australia Grand Prix on March 17.

Hamilton, who replaces Germany's seven-times world champion Michael Schumacher, could have a hard road ahead of him before the regulations change.

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