BULBOACA, Moldova, June 1 (Reuters) - NATO will monitor the skies over Moldova as more than 40 European leaders attend a summit close to Ukraine's borders to show support for both countries as Kyiv prepares a counter-offensive against Russia's invasion.

The gathering of the EU's 27 member states and 20 other European countries at a castle deep in Moldovan wine country just 20 km (12 miles) from Ukrainian territory poses a security and organisational challenge for a country of 2.5 million people wedged between Ukraine and NATO member state Romania.

NATO Airborne Warning and Control Systems (AWACS) surveillance aircraft will watch the skies over the summit venue through Friday, the alliance said in a statement.

Missile debris from the war in Ukraine has been found in Moldova several times since Russia invaded 15 months ago.

"NATO AWACS can detect aircraft, missiles and drones hundreds of kilometres away, making them an important early warning capability," NATO spokesperson Oana Lungescu said.

With Kyiv preparing for a counteroffensive using recently acquired Western weapons to try to drive out Russian occupiers, much of the summit's focus will be on Ukraine.

Ukrainian President Volodymyr Zelenskiy was invited to the summit.

"The presence of these leaders in our country is a clear message that Moldova is not alone and neither is our neighbour Ukraine, which for a year and three months has been standing against the barbaric invasion of Russia," President Maia Sandu told reporters alongside European Commission President Ursula von der Leyen.