By Humeyra Pamuk and Kemol King

GEORGETOWN, March 27 (Reuters) - It would be a "a very

bad day" for Venezuela if it were to attack its neighbor Guyana

or oil giant ExxonMobil U.S. Secretary of State Marco

Rubio said on Thursday during his visit to Guyana's capital.

Guyana and Venezuela are locked in a long-running dispute

about which country has rights over the 160,000-square-km

(62,000-square-mile) Esequibo area, which is the subject of an

ongoing case at the International Court of Justice (ICJ).

Tensions rose early this month when Guyana said a Venezuela

coast guard patrol entered its waters and approached an output

vessel in an offshore oil block operated by ExxonMobil ( XOM ).