By Ed Stoddard

MARIKANA, South Africa (Reuters) - Labor strife in South Africa's platinum belt intensified on Wednesday as a wildcat strike across Lonmin's shafts entered a second day, raising fears that further violence could hit the country's credit rating.

Tensions have been running high over looming job cuts and wage talks in the sector, complicated by a turf war between rival unions that contributed to violent strikes at Lonmin and other platinum producers last year.

Further outbreaks of violence could damage the export competitiveness of South Africa's mining industry, with a possible credit downgrade for Africa's biggest economy, analysts at ratings agency Moody's said in Johannesburg on Wednesday.

At Lonmin's Marikana mine 120km (70 miles) northwest of Johannesburg, several dozen strikers brandishing sticks and branches marched to a dusty football pitch on Wednesday, chanting slogans denouncing the security forces.

Company spokeswoman Sue Vey told reporters that Lonmin had not been issued with any formal demands relating to the two-day walkout at all its 13 shafts. "It seems to be union rivalry," she said.

The rivalry between the Association of Mineworkers and Construction Union (AMCU) and the National Union of Mineworkers (NUM) erupted early last year at rival Impala Platinum.

The AMCU has recruited thousands of members from the 30-year-old NUM. Since the start of the unrest, violent strikes and protests have resulted in more than 50 deaths, including the killing of 34 striking Marikana miners by police last August.

An NUM spokesman said on Tuesday that the latest strike appeared to stem from anger over the killing of an AMCU member in a Rustenburg tavern on Saturday.

Police kept a low profile on Wednesday, with only one mine security guard watching as the striking workers made their way to the football pitch.

"The police are shivering," shouted the marchers, wearing the signature emerald green shirts of the AMCU.

OMINOUS SIGN

The challenge to NUM's dominance of the sector has also rattled the African National Congress (ANC), the mining union's ally in the struggle against apartheid, especially as the ruling party gears up for an election this time next year.

The previous day, strikers warned that those wanting to return to work would be treated like "rats", an ominous sign of intimidation that has prevailed around Marikana since August.

"We don't want NUM," one miner told Reuters, declining to give his name.

The hostility between the factions on the ground was mirrored in a fractious radio debate on Wednesday, in which the two unions' leaders traded insults and blamed each other for instigating the violence that undermined 2012 growth and triggered ratings downgrades early this year.

"Their introduction in the area was through violence," NUM general secretary Frans Baleni said on the Talk Radio 702 show.

"People were intimidated to join AMCU. They were threatened and others were killed. For them to sustain that membership, they must maintain instability, severe violence, otherwise people will come back to the NUM."

AMCU president Joseph Mathunjwa countered by accusing senior ANC party deputy president and NUM founder Cyril Rampahosa of stoking the fire in a May Day speech in which he urged the NUM to fight back.

Lonmin shares fell more than 3 percent, compounding Tuesday's 7 percent decline, while the rand retreated to a three-week low against the dollar on fears that the unrest might spread across a sector that accounts for nearly half of South Africa's foreign exchange earnings.

(Writing by Sherilee Lakmidas; Editing by Ed Cropley and David Goodman)