A Chinese Navy submarine takes part in an international fleet review to celebrate the 60th anniversary of the founding of the People's Liberation Army Navy in Qingdao, Shandong province April 23, 2009 POOL New/REUTERS The US is worried that it's submarine fleet is falling behind China and Russia's as both countries aggressively push to expand and modernize their navies.

Admiral Harry Harris told the House Armed Services Committee on Wednesday that US Pacific Command “suffers shortage of submarines today, my requirements are not being met."

And, according to Foreign Policy, Harris told the committee that his main concern was that the Pentagon was not managing to keep pace with its rivals in the region.

Harris directly linked this threat to the rising submarine powers of Russia and China.

Russia already has the second most capable submarine fleet in the world after the US, Harris admitted, and he warned that their capabilities were likely to rise in the coming years as the US submarine force falters.

Currently, Foreign Policy notes, the US Navy's shipbuilding plan calls for a fall in the number of attack submarines from 52 to 41 by 2028 before "gradually clawing back to 50 by 2044."

As the number of US submarines is expected to fall, China and Russia are expanding their submarine forces. Russia has expanded a naval base to allow for new ballistic-missile submarines in the Northern Pacific.

At the same time, Moscow has also undertaken a serious push for the expansion and modernization of its submarine fleet — including the introduction of new submarine models in both Europe and Asia.

This expansion of the Russian submarine fleet includes more aggressive submarine posturing throughout the world.

The US had to recently announce plans to reopen a submarine base in Iceland following reports of Russia operating submarines in the North Atlantic at Cold War-era levels.

A Russian submarine AP Photo/ Dmitry Lovetsky

And while China's submarine fleet is nowhere at the same level of capability of the US or Russia, it too is being rapidly built up. Foreign Policy reports that China has built four new ballistic missile submarines, and is thought to be constructing an unknown number of additional vessels.

This construction boom comes amid the US pivot to the Pacific and China's increasing militarization of the South China Sea. Most recently, China has based missiles on a contentious island that China, Taiwan, and Vietnam all claim.

Amid these ongoing threats, US forces are facing serious strain as they must operate on multiple fronts in both the Pacific and the Atlantic. And as the US submarine force shrinks, and Russia and China's continue to grow, US forces will likely come under greater and greater stress.