Image caption The National Weather Service has placed nearly 20 states throughout the US under a heat warning, watch or advisory

A heat-wave blanketing the US heartland in humidity has claimed the lives of 13 people, according to US media.

The National Weather Service put 18 states stretching from Montana to Texas to West Virginia under a heat warning, watch or advisory, with the heat index topping 38C (100F) in most locations.

The heat is expected to move east in the next several days.

More than 1,000 US heat records have been broken this month, officials said.

Though many US states have recently seen temperatures over 90F, some regions saw heat indexes - a combination of air temperature and relative humidity - up to 131F.

Buckling roads

During the next several days, a "combination of very hot temperatures and high humidity will create dangerous heat indices over the central US", said the National Oceanic and Atmospheric Administration (NOAA).

US heat indexes Knoxville, Iowa: 131

Freeport, Illinois: 124

Hutchinson, Minnesota: 123

Watertown, Wisconsin: 119

Tekamah, Nebraska: 117 Source: NOAA

The National Weather Service said a stagnant air mass on the central plains was causing the extended heat-wave, which has plagued swathes of the US for nearly a week.

The US Border Patrol said high temperatures were also responsible for a spike in deaths of illegal immigrants trying to cross the border from Mexico.

Roads in Oklahoma were reported to be cracking due to the heat, while dozens of the state's residents have required medical treatment in the past several days.

On Monday, Oklahoma experienced its 28th day of triple-digit temperatures this year, The Oklahoman newspaper reported.

An extreme drought that has also afflicted several south-west states in recent weeks caused water-main ruptures to quadruple in Oklahoma City to up to 20 per day, city officials said.

Meanwhile, on Tuesday, Illinois baked in temperatures that soared over 110F, while the town of Knoxville in Iowa experienced heat indexes over 130F, the National Weather Service said.

Residents in Minnesota, a mostly temperate state, saw heat indexes reach 122F.

NOAA released data in June showing that temperatures had risen across the US by roughly 1.5F during the past 30 years.

Forecasters said unusual heat would continue to through to the end of the month.

Heat is responsible for killing 162 people in the US on average each year.