LONDON (Reuters) - Iran has said it would block the Strait of Hormuz if it was barred from using the strategic waterway through which about a fifth of oil that is consumed globally passes.

Oil tankers pass through the Strait of Hormuz, December 21, 2018. REUTERS/Hamad I Mohammed

The threat from an Islamic Revolutionary Guards Corps (IRGC) commander followed a U.S. announcement on Monday that it would end exemptions granted last year to eight buyers of Iranian oil and demanding they stop purchases by May 1 or face sanctions.

Oil prices have surged to six-month highs.

The Strait of Hormuz, a vital shipping route linking Middle East oil producers to markets in Asia, Europe, North America and beyond, has been at the heart of regional tensions for decades.

Iran has made threats to block the waterway in the past, without acting on them.

Below is some background about the Strait:

WHAT IS THE STRAIT OF HORMUZ?

WHY DOES IT MATTER?

ARE THERE ALTERNATIVE ROUTES FOR GULF OIL?

HAVE THERE BEEN INCIDENTS IN THE STRAIT BEFORE?

Sources: Reuters/Refinitiv/Energy Information Administration