UPDATE 1-UPS offers more concessions to secure TNT deal-sources

* Latest concessions are substantially improved - source

* First offer included selling warehouses, customer bases

BRUSSELS, Dec 18 (Reuters) - United Parcel Service Inc has offered more concessions in a bid to win EU regulatory approval for its 5.2-billion-euro ($6.8 billion) bid for TNT Express, two people familiar with the matter said on Tuesday.

The latest offer came after the European Commission indicated an earlier proposal to sell warehouses and customer bases in about 15 countries, mainly in eastern Europe, was not sufficient to address competition concerns.

The world No. 1 package delivery company is seeking to buy Dutch peer and European market leader TNT to gain access to TNT's network in the fast-growing Asian and Latin American markets.

UPS submitted its latest offer to the European Commission on Monday, one of the sources said.

"The remedies are substantially improved," the second person said, declining to provide details because of the sensitivity of the matter.

The EU antitrust authority, which will decide by Feb. 5 whether to clear or block the deal, has told UPS rivals and customers to provide feedback on the latest concessions by Wednesday, one of the people said.

Both the Commission and UPS declined to comment. TNT shares added gains after Reuters reported the latest UPS concessions and were up 3 percent to 8.18 euros in mid-session.

UPS' earlier offer was aimed at boosting French mail group DPD as a viable third European competitor able to offer both road and air freight services.

The EU watchdog is concerned UPS's takeover of TNT would result in the merged entity competing mainly with just Deutsche Post's DHL unit in Europe. The German package delivery company has a 15 percent share of the European market, behind TNT's 18 percent.

UPS also competes with U.S. rival FedEx.