SINGAPORE - Extreme heat across large tracts of the Northern Hemisphere raised fears for crops in China, fueled forest fires in Portugal and Russia's Far East, forced flight cancellations in the Southwest US and melted pavement on roads in Britain.

Wednesday marked the summer solstice-the longest day of the year-and forecasters said temperatures in Paris were expected to hit 37 C, Madrid could see 38 C and London was set for 34 C with warnings of thunderstorms.

Rounding up the record temperatures set in the past two months, the World Meteorological Organization said Earth was experiencing "another exceptionally warm year" and the heat waves were unusually early.

"Parts of Europe, the Middle East, North Africa and the United States have seen extremely high May and June temperatures, with a number of records broken," the organization said on Tuesday.

The trend seen during the past two months has put average monthly global temperatures among the highest ever recorded since data began to be collected in 1880.

Even before this month, US National Oceanic and Atmospheric Administration data showed that Europe, the US and Northeast Asia - including eastern China, Japan and South Korea - had experienced unusually warm weather between March and May.

In China, the world's top grain producer, hot and dry conditions in the main corn belt have delayed plantings and stunted crop development, especially in Liaoning province, where soil moisture levels are at their lowest in at least five years.

"The drought that hit parts of China's northeast is the worst for this time of the year in the past decade, in the breadth of areas it has affected and the length of time it has lasted," said Ma Wenfeng, analyst at Beijing Orient Agribusiness Consultancy.

Reuters