The United States has once again ramped up trade pressure on key partners by threatening punitive tariffs on imports from 14 countries while simultaneously extending a temporary grace period for which so-called "reciprocal" tariffs negotiation.

U.S. President Donald Trump said on Monday on social media that 25 percent tariffs will be imposed on imports from Japan and the South Korea, respectively, beginning on August 1.

Later, he announced that letters had been sent to the leaders of 12 other countries, namely Malaysia, Kazakhstan, South Africa, Myanmar, Laos, Tunisia, Indonesia, Bangladesh, Serbia, Bosnia and Herzegovina, Cambodia and Thailand, informing them that tariffs ranging from 25 percent to 40 percent will be charged starting next month.

The planned tariffs are on:

Japan, South Korea, Kazakhstan, Malaysia and Tunisia: 25 percent

Bosnia and Herzegovina and South Africa: 30 percent

Indonesia: 32 percent

Bangladeshi, Serbia: 35 percent

Cambodian, Thailand: 36 percent

Laos, Myanmar: 40 percent

In the almost identical letters, Trump asked leaders of the respective countries to understand that the threatened tariff rate "is far less than what is needed to eliminate the Trade Deficit disparity we have with your Country."

The letters also contained a warning: if the countries raise tariffs in response, the United States will increase its tariffs by the same amount.

The U.S. administration justifies the tariffs on grounds of economic and national security, framing them as essential to correcting longstanding trade imbalances. Trump reiterated that the measures are separate from industry-specific tariffs and warned against using third-country rerouting to avoid duties, which he said would result in even higher penalties.

U.S. Treasury Secretary Scott Bessent signaled further pressure over the weekend, suggesting that countries failing to reach an agreement by August 1 would face the full scale of the reciprocal tariffs announced in April.

"If you want to speed things up, have at it. If you want to go back to the old rate, that’s your choice," Bessent told CNN on Sunday, while refusing to confirm whether August 1 would be the final cut-off date.

Several affected countries have publicly voiced frustration over the process:

South Africa said it would continue pushing for a "more balanced and mutually beneficial" trade relationship through diplomatic channels.

European Commission President Ursula von der Leyen said the EU was ready for a deal "in principle" but warned of firm countermeasures if talks collapse.

Japanese Prime Minister Shigeru Ishiba said Tokyo would firmly defend its interests and is prepared for "all tariff contingencies."

South Korean President Lee Jae-myung admitted that talks remain difficult and "both sides are unsure of what they want."

India notified the World Trade Organization of planned retaliatory tariffs on certain U.S. goods, citing unfair duties on its automobile exports.

Vietnam reiterated calls for the U.S. to recognize its market economy status and lift high-tech export restrictions.

News of the tariff letters sent shockwaves through financial markets. On Monday, U.S. equities fell sharply amid investor fears of a renewed global trade conflict.

The Dow Jones Industrial Average dropped 422.17 points or 0.94 percent. The S&P 500 fell 49.37 points or 0.79 percent. The NASDAQ Composite Index is down 188.59 points or 0.92 percent.

Analysts warned that another escalation in trade tensions could revive the volatility seen earlier this year, when markets tumbled amid uncertainty over global supply chains and protectionist policies.