As corporations began struggling amid the coronavirus pandemic, U.S. banks' balance sheets started to swell at the end of the first quarter, including their exposure to foreign countries.

Foreign exposure at U.S. banks reached approximately $4.5 trillion during the first quarter, up $691.56 billion or 18.2% from the previous quarter, according to the Federal Financial Institutions Examination Council's E.16 Country Exposure Lending Survey. The exposure figure includes any cross-border claims such as loans made to foreign-based borrowers, securities or credit derivatives. As of March 31, U.S. banks' foreign exposure represented 23.1% of total claims, up from 22.0% at year-end 2019.

U.S. banks reported the highest exposure to the U.K, Cayman Islands, Japan, Germany and France, with Cayman Islands and France recording the highest increases of $133.71 billion and $113.91 billion, respectively. Roughly 37% of the total increase in France came from BNP Paribas USA Inc.

Meanwhile, the industry's exposure to Mexico declined $7.57 billion to $93.83 billion, thanks largely to Citigroup Inc. Citi represented 70.3% of U.S. banks' total exposure to Mexico as of March 31, even after its exposure to the country dropped $7.72 billion during the quarter.

During Citi's first-quarter earnings call, CFO Mark Mason said the bank is seeing slower spending patterns in Mexico, similar to Asia, which will likely put pressure on loan balances.

"And while we haven't yet seen the full impact from COVID-19 in Mexico, we did see a slowdown in purchase sales in March, which is expected to continue as customer behavior will likely follow the pattern we've seen in other regions," Mason said.

Citi CEO Michael Corbat said the bank is taking a country-by-country approach, looking at the response to the health crisis, fiscal and monetary policy and underlying demographics. He said Citi is watching developments in Mexico closely.