FRANKFURT, Germany (AP) — Fears are rising about what would happen to Europe's energy supply if Russia were to invade Ukraine and then shut off natural gas exports in retaliation for U.S. and European sanctions.

The tensions show the risk of Europe's reliance on Russia for energy, which supplies about a third of the continent's natural gas. And Europe's stockpile is already low. While the U.S. has pledged to help by boosting exports of liquefied natural gas, or LNG, there's only so much it can produce at once.

It leaves Europe in a potential crisis, with its gas already sapped by a cold winter...