BERLIN, March 3 (Reuters) - Germany has asked Switzerland to sell it some of its mothballed Leopard 2 tanks, the Swiss and German governments said on Friday, in a deal that could allow Western countries to increase military aid to Ukraine.

Germany wants Switzerland to sell some of the tanks back to arms maker Rheinmetall (RHMG.DE), which would allow the company to backfill gaps in the armaments of European Union and NATO members.

Germany, Poland, Portugal, Finland and Sweden are among countries sending Leopard tanks to help Ukraine defend itself against Russian attack, creating gaps in their own arsenals.

German Defence Minister Boris Pistorius and Economics Minister Robert Habeck informed Swiss Defence Minister Viola Amherd about the project in a letter dated Feb. 23, the defence ministries in Bern and Berlin told Reuters on Friday.

The story was first reported by Swiss newspaper Blick.

In the letter seen by Reuters, the German government said it would be very grateful if Switzerland could approve the purchase of mothballed Leopard 2 tanks by Rheinmetall, as it had done in the past, if those tanks were not intended to be put into operation again.

"The tanks will not be sold on to Ukraine. We guarantee that they will remain in Germany or with our partners in NATO and the EU, to plug gaps that have emerged through the transfer of Leopard 2 tanks and to improve the supply with spare parts in general," the letter said.

It did not detail how many tanks Berlin was aiming for but it acknowledged that the German government was aware "that there is a discussion in Switzerland, too, about the consequences (of Russia's war in Ukraine) for its national defence".

Under its neutrality laws and a separate arms embargo, Switzerland is prohibited from sending weapons directly to Ukraine.

Swiss Defence Minister Viola Amherd replied in a letter on March 1 that a possible sale of part of the Swiss tank fleet would require the Swiss parliament formally declare the mothballed tanks to be out of service, the VBS said.

"Discussions on this issue are currently under way in parliament," the VBS spokesman added.

The Swiss military currently has 134 Leopard 2 tanks in service and a further 96 in storage. The government did not say how many Germany had requested to buy, but the VSB spokesman said the army had said it would be possible to dispense with a limited number of tanks.

No information was given when the Swiss parliament could decide on the issue, although the government said it would comment further on Monday.

Bern has previously blocked requests from Germany, Spain and Denmark to allow Swiss-made munitions they have previously bought to be re-exported to Ukraine.

But the issue is becoming increasingly divisive in Switzerland, with a pro-Ukraine shift in the public and political mood putting pressure on the government to end a ban on exports of Swiss weapons to war zones.

Calls from Switzerland's European neighbours to allow such transfers to Kyiv have grown louder, and parliament's two security committees have recommended that the rules be eased.