Syria's internet and phone lines started working again this evening, residents said, after a day-long blackout left much of the population cut off from the outside world.

Communication networks have been crucial for opposition activists trying to get out videos and information about a two-year conflict that has killed more than 70,000 people.

Activists called the outage an intentional move to aid countrywide military operations.

Google's Transparency Report pages showed traffic to Google services pages from the country suddenly stopping shortly before 8pm Irish time yesterday.

It returned at 3.15pm Irish time today.

Syrian state news agency SANA quoted the Director General of the General Establishment for Communications Bakr Bakr as saying that internet services and communication between provinces had gone down due to a malfunction in an optic cable.

But the Syrian Observatory for Human Rights, a British-based group that monitors both President Bashar al-Assad's security forces and the rebels, cited military sources who said the blackout was part of a security force operation.

The insurgency against four decades of Assad family rule began as peaceful protests but descended into civil war after months of fierce crackdown by security forces.

Mr Assad's forces have often shut down telephone and internet connections in some cities or neighbourhoods during major combat operations.

Opposition activists accuse them of doing the same during alleged massacres that killed hundreds of people.

Most recently, communications appeared to have been cut in the coastal towns of Banias and Baida during what activists said were widespread executions that killed hundreds of people, including dozens of children.

Speaking before connections were restored today, Christine Chen, Google's senior manager for free expression, said: "We've seen this twice before.

"This happened in Syria last November and in Egypt during the Arab Spring."

Google's data showed traffic disruptions limited to Syria and spanning the entire country.

It is virtually impossible to definitely determine the cause of such disruptions unless a party claims responsibility, experts said.

Shutting an entire nation from the internet is possible because IP addresses, individual connections established by each device, are geographically specific and the government has control over the country's internet service providers.

The vast majority of websites within Syria were rendered unreachable as well, other experts said, as the county appeared to shut itself off.