"The world just had the hottest week on record, according to preliminary data," the WMO said in a statement, after climate change and the early stages of the El Nino weather pattern drove the warmest June on record.
It's the latest in a series of records halfway through a year that has already seen a drought in Spain and fierce heat waves in China as well the United States.
Temperatures are breaking records both on land and in the oceans, with "potentially devastating impacts on ecosystems and the environment", the WMO said.
"We are in uncharted territory and we can expect more records to fall as El Nino develops further and these impacts will extend into 2024," said Christopher Hewitt, WMO Director of Climate Services.
"This is worrying news for the planet."
The WMO said it had looked at various datasets from partners around the world.
Europe's climate monitoring service Copernicus told AFP its data also showed last week was likely to be the hottest since records began in 1940.
Copernicus said that its data suggests Thursday was likely to have seen the highest global average temperature, after several record-breaking days earlier in the week.
'Out Of Control'
Last week the Canadian Ministry of Natural Resources said the number of wildfires in the country, more than 670 on Friday, was "off the charts" with a long and difficult summer ahead.
Smoke from the fires so far this season has fouled the air in Canada and neighbouring United States, affecting more than 100 million people.
In the US, Texas is experiencing a prolonged "heat dome" in which warm air is trapped in the atmosphere like a convection oven, while in Europe, Spain is bracing for its second heatwave in a matter of weeks.
In southern Iraq, the fabled marshland is suffering its worst heatwave in the past 40 years, the UN's Food and Agriculture Organization said on Monday, warning of a "devastating impact" on the ecosystem as well as local farmers and fisheries.
UN Secretary General Antonio Guterres has said "the situation we are witnessing now is the demonstration that climate change is out of control".
As well as withering crops, melting glaciers and raising the risk of wildfires, higher-than-normal temperatures also cause health problems ranging from heatstroke and dehydration to cardiovascular stress.
New research published Monday found that more than 61,000 people died due to the heat during Europe's record-breaking summer last year.
The majority of deaths were of people over the age of 80 and around 63 percent of those who died due to the heat were women, according to the research published in the journal Nature Medicine.
The world has warmed an average of nearly 1.2 C since the mid-1800s, unleashing extreme weather including more intense heatwaves, more severe droughts in some areas and storms made fiercer by rising seas.
Oceans absorb most of the heat caused by planet-warming gases, causing heatwaves that harm aquatic life, altering weather patterns and disrupting crucial planet-regulating systems.
In June, global sea surface temperatures have hit unprecedented levels, while Antarctic sea ice reached its lowest extent for the month since satellite observations began, at 17 percent below average, breaking the previous June record by a substantial margin.
While sea surface temperatures normally recede relatively quickly from annual peaks, this year they stayed high, with scientists warning that this underscores an underappreciated but grave impact of climate change.
"If the oceans are warming considerably, that has a knock-on effect on the atmosphere, on sea ice and ice worldwide," said Michael Sparrow, chief of World Climate Research Programme at the WMO.
"There's a lot of concerns from the scientific community and a lot of catch-up from the scientific community trying to understand the incredible changes that we're seeing at the moment."
El Nino is a naturally occurring pattern that drives increased heat worldwide, as well as drought in some parts of the world and heavy rains elsewhere.
But Sparrow said its effects would likely be felt more acutely later in the year.
"El Nino hasn't really got going yet," he said.