Two hurricanes are expected to slam into the US Gulf Coast in the coming days, forecasters said Sunday, as Tropical Storm Laura killed at least 12 people when it struck Haiti and the Dominican Republic.

US media said twin hurricanes were unprecedented in the Gulf of Mexico since records began 150 years ago. 

Tropical Storm Marco strengthened into a hurricane with winds of 75 miles (120 kilometers) per hour, and is forecast to hit the state of Louisiana on Monday.

Tropical Storm Laura hammered Haiti and the Dominican Republic with heavy rain, killing at least 12 people -- 9 in Haiti and three in the Dominican Republic. 

It was set to become a hurricane on Tuesday that could hit the US coastal region on Wednesday. Energy companies have suspended some oil and natural gas production in the Gulf as the weather deteriorates.

The US National Hurricane Center said Storm Laura was "bringing torrential rainfall and life-threatening flooding" to Haiti, which shares Hispaniola island with the Dominican Republic. The storm killed three people in the Dominican Republic's capital Santo Domingo, said Juan Manuel Mendez, head of the country's Center of Emergency Operations.

A woman and a child died at home, while a young man died when a tree fell on his home, Mendez said. The storm has flooded houses, cut off remote villages and left more than one million Dominicans in the dark, Mendez added.

In neighbouring Haiti, a 10-year-old girl was among the nine dead, authorities said. They said some homes were flooded and evacuations were underway.

Flooded homes 

With brown water up to their knees, some residents tried to save what they could from their flooded homes, while street traders saw their goods washed away.

"I didn't know there was bad weather forecast. We don't often have electricity in my neighbourhood so I couldn't follow the news on the radio," said Sony Joseph, trembling with cold.

The Atlantic storm season, which runs through November, could be one of the busiest ever this year, with the Hurricane Center predicting as many as 25 named storms. Laura is the 12th so far.

Haiti, a country of 11 million, has seen a relatively low incidence of COVID-19 -- with just over 8,000 cases and around 200 deaths to date -- but authorities urged caution to prevent further spread in the aftermath of Storm Laura.

"Wear your masks and respect distances, especially in temporary shelters," Interior Minister Audain Fils Bernadel said at a briefing Saturday. "With COVID, we have considerably less capacity in our shelters."

Storms pose a serious risk to Haiti every year from June to November. Even a heavy rainfall can threaten the country's poorest residents, many of them living in at-risk zones, near canals or ravines that can be obstructed by debris and quickly overflow.

The Miami-based Hurricane Center said on Sunday that "a slightly stronger Laura was just south of eastern Cuba" after sending "life-threatening flash flooding likely over the Dominican Republic, Haiti, Cuba and Jamaica." 

Marco was expected to bring "life-threatening storm surges and hurricane force winds" to parts of the US Gulf coast on Monday and Tuesday, the NHC warned.