Iran and Israel launched new missile strikes at each other on Wednesday as the air war between the two longtime enemies entered a sixth day despite a call from US President Donald Trump for Tehran's unconditional surrender.

The Israeli military said two barrages of Iranian missiles were launched towards Israel in the first two hours on Wednesday morning. Explosions were heard over Tel Aviv.

Israel told residents in a southwestern area of Tehran to evacuate so its air force could strike Iranian military installations. Iranian news websites said Israel was attacking a university linked to Iran's Revolutionary Guards in the east of the capital.

Iranian news websites said Israel was also attacking a university linked to Iran's Revolutionary Guards in the country's east, and the Khojir ballistic missile facility near Tehran, which was also targeted by Israeli air strikes last October.

The US office of the director of national intelligence said Iran is armed with the largest number of ballistic missiles in the Middle East. Iran has said its ballistic missiles are an important deterrent and retaliatory force against the US, Israel and other potential regional targets.

Trump warned on social media on Tuesday that US patience was wearing thin. While he said there was no intention to kill Iran's leader Ayatollah Ali Khamenei “for now”, his comments suggested a more aggressive stance towards Iran as he weighs whether to deepen US involvement.