Why did Lebanese security forces beef up presence around Swedish embassy in Beirut?

An assailant threw a Molotov cocktail at Sweden's embassy in Beirut, causing no casualties, Stockholm's foreign minister and a diplomatic source said on Thursday, amid anger over recent Quran desecrations.

"We confirm that there was a Molotov cocktail thrown at the facade of our embassy last evening, which did not explode," a diplomatic source at the embassy said, requesting anonymity as they were not authorised to talk to the media.

The "perpetrator managed to run away", the source added.

Tensions have flared between Sweden and Muslim countries following several protests involving public desecrations of the Quran in Stockholm - including setting pages alight.

Multi-confessional Lebanon saw protests at mosques, while the head of the powerful pro-Iran Shia Muslim group Hezbollah has called for the Swedish ambassador to be expelled.

Lebanese security forces had bolstered measures around the embassy in downtown Beirut for fear of attacks.

Swedish Foreign Minister Tobias Billstrom said "it was sheer luck that no one was injured" in Wednesday's attack and that staff were safe.

"The incident is currently being investigated," he said in a statement on Thursday, noting that "Lebanese authorities have an obligation under the Vienna Convention to protect diplomatic missions".

Late last month, two Iraqi men set a copy of the Koran alight outside the Swedish parliament in Stockholm, in an act similar to others in recent weeks that had drawn widespread condemnation.

Salwan Momika and Salwan Najem stomped on the Muslim holy book, set its pages ablaze before slamming it shut, as they did at a protest outside Stockholm's main mosque in June.

The duo also staged a similar protest outside Iraq's embassy in the Swedish capital on July 20, where they stomped on the religious text.

Iraqi protesters had stormed the Swedish embassy in Baghdad twice, starting fires within the compound on the second occasion.