The White House says recent encounters between Iranian vessels and U.S. warships in the Persian Gulf are a serious cause for concern.

Earlier this week, the Pentagon confirmed that the U.S. Navy fired warning shots towards Iranian sea vessels that approached two U.S. ships at high-speeds in the Strait of Hormuz. 

Iran's defense minister said the Iranian patrol ships were just doing their job. Tensions have increased between Iran and the US in recent days, despite an overall improvement in bilateral relations. Serious differences still remain over a number of issues including -- Iran's ballistic missile program, the Syrian conflict and anti-terrorism efforts in Iraq.