The Paris-based economic think-tank said its composite leading indicator (CLI) for China, which provides a measure of future economic activity, slipped to 99.1 from 99.4 in April, falling further below its long-term average of 100.

The CLI for India also showed signs of weakening, dropping to 98.0 from 98.2, again below the 100 average.

"The assessment for China and India has changed significantly since last month. For both countries, the CLIs point towards economic activity below long-term trend," the Organisation for Economic Co-operation and Development said in its report.

The overall CLI for the OECD area, covering 33 countries, inched up to 100.5 from 100.4 over the month, helped by fresh growth in activity in the United States, Japan and Russia.

But the OECD said the pace of improvement in all three countries had decelerated in recent months, giving a tentative sign their growth may be about to slow.

The euro area, meanwhile, remained stable at 99.6, as did France, while Italy inched down to 99.1 from 99.2, as ongoing debt troubles in Europe continued to keep economic growth below long-term trend.

The CLI for Germany was also unchanged at 99.4. Britain's, however, inched up to 99.8 from 99.7 but remained below its long-term average.

(Reporting By Vicky Buffery; Editing by Toby Chopra)

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