NEW YORK, June 18 (Reuters) - Foreigners bought an estimated $103 billion of U.S. long-term securities in April while boosting their holdings of Treasury securities by $4 billion, the Treasury said on Thursday in its monthly Treasury International Capital report. 

Among the largest holders of Treasury securities, Japan boosted its holdings to $1.21 trillion in April from $1.19 trillion a month earlier, the report said.

UK holdings rose to $938 billion from $927 billion in February while China's holdings dropped to $651 billion from $652 billion.

The rise in Treasury holdings put total foreign holdings at $9.353 trillion, up from March's level but down from February's record of $9.49 trillion. 

The report comes at a time when investors are closely scrutinizing factors such as foreign demand for U.S. debt given the Federal Reserve's ongoing battle with inflation and a roaring bull market in AI-related stocks.

On Thursday, a U.S. auction of 5-year Treasury Inflation-Protected Securities was well received, reflecting in part the rise in recent weeks of inflation-adjusted "real rates" in the TIPS market. 

The total in April of all net foreign acquisitions was a net TIC inflow of $26.1 billion, Treasury said. Of this, net foreign private outflows were $23.1 billion and net foreign official inflows were $49.2 billion.

Foreign residents increased their holdings of long-term U.S. securities in April, the Treasury added, with net purchases of $206 billion. Net purchases by private foreign investors were $164.4 billion and net purchases by foreign official institutions were $41.6 billion.