The European Union has a crucial role to play in diffusing military tensions between the United States and Iran, the bloc's top diplomat said, after the U.S. Defense Department insisted that an American surveillance drone shot down by an Iranian projectile in the Gulf region on Wednesday was operating over international waters.

Federica Mogherini, the EU's High Representative for Foreign Affairs and Security Policy, told CNBC ahead of a gathering of European leaders in Brussels Thursday afternoon that Europe will try to "open channels of communication and make sure that an escalation is avoided," as oil prices spiked significantly over widespread supply fears.



President Trump said in a Twitter post that Iran had made a "very big mistake" in its decision to shoot down the RQ-4A Global Hawk, an unarmed, unmanned but highly advanced high-altitude surveillance aircraft. Later Thursday, Trump seemed to downplay the incident, suggesting that it may have been unintentional.



This followed an earlier claim from Iranian state-run broadcaster Press TV that the country's Revolutionary Guard had successfully brought down an "intruding American spy drone."



But according to Navy Capt. Bill Urban, a spokesman for the U.S. military's Middle East regional command known as Centcom, Iran's use of a surface-to-air missile on the Global Hawk represented an "unprovoked attack on a U.S. surveillance asset in international airspace."



In a sign of the seriousness with which the U.S. military is treating this latest incident, the top U.S. Air Force commander in the region, Lt. Gen. Joseph T. Guastella, spoke to Pentagon journalists from the Al Udeid Air Base in Qatar, where around 10,000 US military personnel are permanently stationed.



The United States last week announced it would send an additional 1,000 troops to the Middle East, in addition to the 1,500 extra personnel it promised to dispatch in May.



And senior military commanders have previously warned that Iranian attacks on U.S. forces or interests in the region would prompt a response.



Mogherini, the European diplomat, told CNBC she had agreed with her American counterpart Secretary of State Mike Pompeo earlier this week at a meeting in Washington D.C. that it was not in anybody's interest "to see a military escalation," and her focus remained on keeping Iran compliant with its obligations under the nuclear deal she helped broker, formally known as the Joint Comprehensive Plan of Action or JCPOA.



"We'll try to do what we can to diffuse tensions," she said.



But Sanam Vakil, who heads the Iran forum at the London-based think tank Chatham House, said the drone shooting was the latest in a "cascading series of attacks," which included several that damaged oil tankers in the stretch of water that separates Iran from the Arabian peninsula.



"Iran is increasing its leverage for future negotiations" over sanctions relief and its nuclear capabilities, Vakil told CNBC via email.



"Without dialogue, diplomacy and serious de-escalation, the risk of action and reaction and a slide into a wider regional conflict is significant."