GENEVA (Reuters) -The Trump administration's decision to extend a negotiating deadline for tariff rates is prolonging uncertainty and instability for countries, the executive director of the United Nations trade agency said on Tuesday.

U.S. President Donald Trump on Monday ramped up his trade war, telling 14 nations, from powerhouse suppliers such as Japan and South Korea to minor trade players, that they now face sharply higher tariffs from a new deadline of August 1.

"This move actually extends the period of uncertainty, undermining long-term investment and business contracts, and creating further uncertainty and instability," Pamela Coke-Hamilton, executive director of the International Trade Centre, told reporters in Geneva.

"If a business is not clear on what costs they are going to pay, they cannot plan, they cannot decide on who will invest," Coke-Hamilton said, citing the example of Lesotho, where major textile exporting companies have withheld their investment for the time being, pending a tariff outcome.

The uncertainty, combined with deep cuts in development aid, had created a "dual shock" for developing countries, she added.

Countries have been under pressure to conclude deals with the U.S. after Trump unleashed a global trade war in April that roiled financial markets and sent policymakers scrambling to protect their economies.