South Africa's ruling African National Congress on Friday said its website had been hacked by Zimbabwean activists claiming a link to the global "hacktivist" group Anonymous.

"Someone calling themselves Anonymous and claiming to be the legitimate representative of the people of Zimbabwe has flooded the website of our organisation," the ANC said in a statement.

The denial of service attack -- which floods a website with so many data requests that it crashes -- appeared to be in effect from around 09:00 GMT.

"Our website management team is currently working on the problem, including assessing means to strengthen our security so that such does not recur in future," said spokesman Jackson Mthembu.

Anonymous is a loosely organised group that has been blamed for attacks on the FBI, Visa, MasterCard, the Kremlin, global intelligence firm Stratfor and Sony Pictures Entertainment among others.

The latest hacking attack appears to be linked to South Africa's stance on the ongoing political crisis in neighbouring Zimbabwe.

The South African government has been criticised for its perceived failure to take a hard line against Zimbabwean President Robert Mugabe, the leader of a fellow liberation movement.

Using the Twitter handle "@zim4thewin", a group calling themselves "Anonymous Africa" warned the ANC of the impending attack.

"Tick tock tick tock, your site will stop working in 40 minutes. think about all the blood on your corrupt hands when it is down," the unverified group warned.

A subsequent tweet read: "anc.org.za is tango down! for being corrupt and supporting the mass murdering mugabe #anc #africa #zimbabwe #anonymous"

They said the attack would end at 1000 GMT.

The timing is politically sensitive.

Mugabe on Thursday plunged Zimbabwe back into political crisis by unilaterally announcing elections will be held on July 31.

His political rival Prime Minister Morgan Tsvangirai vowed to fight the decision, arguing that Mugabe wants to avoid reforms and press ahead with a flawed poll to extend his 33-year rule.

The hack also came on the eve of a summit of regional leaders that will decide a response to Mugabe's gambit.

The ANC on Friday defended its role in easing political violence in Zimbabwe, pointing to the establishment of a power sharing government and the passing of a new constitution.

The ANC vowed to "continue to work with the government and people of Zimbabwe to assist them find their own lasting solution to the challenges facing that country."