BRUSSELS (Reuters) - Leaders of the European Union institutions invited Donald Trump to a summit as soon as he can schedule one and cited security and trade ties as key in a letter on Wednesday congratulating him on his election as U.S. president.

"Today, it is more important than ever to strengthen transatlantic relations," wrote Donald Tusk and Jean-Claude Juncker, presidents respectively of the European Council, which groups member states, and the executive European Commission.

"Only by cooperating closely can the EU and the U.S. continue to make a difference when dealing with unprecedented challenges such as Da'esh (Islamic State), the threats to Ukraine's sovereignty and territorial integrity, climate change and migration."

"Fortunately, the EU-U.S. strategic partnership is broad and deep: from our joint efforts to enhance energy security and address climate change, through EU-U.S. collaboration on facing threats to security in Europe's Eastern and Southern neighborhoods and to the negotiations on the Transatlantic Trade and Investment Partnership (TTIP) - we should spare no effort to ensure that the ties that bind us remain strong and durable."

(Reporting by Alastair Macdonald)