However, Morgan is confident that two funds in the region - Legg Mason US Smaller Companies and M&G American - can continue to outperform.
Meanwhile Darius McDermott, managing director at Chelsea Financial Services, recommends AXA Framlington American Growth and JP Morgan US Income for anyone tempted to increase their exposure to the region.
Legg Mason US Smaller Companies
Similar to the UK space, US smaller companies funds can deliver emerging market-type returns, but often with lower volatility.
The £165.9m Legg Mason US Smaller Companies, run by subsidiary Royce & Associates, features on Hargreaves Lansdown's Wealth 150 list of recommended funds.
"It is a fund we like a lot. Royce have tried and tested managers with good records," Morgan said.
"It cannot necessarily be seen as a ‘safe’ investment as they are 100 per cent in equities so investors should be aware of this."
The fund has delivered second-quartile returns of 92.85 per cent since launch in April 2004, compared with 86.02 per cent from the IMA North American Smaller Companies sector average.
The fund also outperformed its Russell 2000 benchmark, which delivered just 72.23 per cent over the period.
Performance of fund vs sector and index since April 2004
Source: FE Analytics
The fund is relatively diversified, with 91 holdings. Its highest sector weighting is in basic materials, with commodity manufacturers Graftech International and Reliance Steel & Aluminum in its top-10.
M&G American
The £951.8m M&G American fund, headed up by FE Alpha Manager Aled Smith, has outperformed the IMA North America sector and the S&P 500 index over long periods of time.
Since Smith took over in 2004, it has returned 67.77 per cent, compared with 52.46 per cent and 41.70 per cent respectively from the sector and benchmark.
Morgan said: "We do like the M&G American fund as it has a good long-term record. It is not the most consistent but it is one of the best large cap focused funds in the region that UK investors can get hold of."
The fund is considerably underweight the tech industry, which accounts for less than 20 per cent compared with 26 per cent from the sector average.
It has returned 163.93 per cent to investors over 10 years, making it a top-quartile performer. However, it has struggled a little in the short-term.
M&G American has a total expense ratio (TER) of 1.67 per cent.
AXA Framlington American Growth
The £647.5m Axa Framlington American Growth features on the FE Select 100 list of recommended funds and is one of McDermott’s picks for US exposure.
"I like AXA Framlington American Growth because of its experienced manager, Stephen Kelly, who has been running the fund for the past decade or so," he commented.
"He has always said he favours growth in the US markets. He believes that if you are looking for value you go to Asia, but over a long-term period, there is a strong growth strategy in the US. Kelly’s cyclical growth strategy has performed well over recent years as well."
The four crown-rated fund has delivered solid top-quartile returns over the long-term, beating the IMA North America sector over three, five and 10 years.
The AXA Framlington American Growth fund has a particularly good record over three years, returning 48.05 per cent – making it the fourth-best performer in the sector over that period – compared with 33.38 per cent from the IMA North America sector average.
Performance of fund vs sector over 3-yrs
Source: FE Analytics
The fund is heavily weighted in favour of telecoms and technology stocks, with an exposure of close to 40 per cent. Blue chips such as Apple, Google and Amazon feature in its top-10 holdings.
JP Morgan US Equity Income
The five crown-rated JP Morgan US Equity Income fund, co-managed by Jonathan Simon and Clare Hart, has outperformed the IMA North America sector and the S&P 500 index over one and three years.
Simon and Hart have headed up the fund since its launch in December 2008. Since this time it has returned 62.74 per cent compared with 58.12 per cent and 57.18 per cent from the sector and benchmark respectively.
"Income has been a successful strategy in the US for a number of years," said McDermott.
"The US equity income index itself has not necessarily been paying high yields. However, the JP Morgan fund has exposure to good-quality globally focused companies that are paying high dividends to their investors and are still at low valuations."
The fund has a 22 per cent exposure to financials, which is almost twice the sector's average.
JP Morgan US Equity Income is currently yielding 2.34 per cent and has a TER of 1.68 per cent.
Baillie Gifford American
Featuring on the FE Select 100 list of recommended funds, the £261.6m Baillie Gifford American has delivered top-quartile performance over three and five years, returning 38.81 per cent over the longer period compared with an IMA North America sector average of 23.63 per cent.
FE Alpha Manager Mick Brewis has been running the fund for 15 years. Over 10 years, he has returned 82.79 per cent, outperforming his peer group composite by more than 15 percentage points.
The fund’s highest weightings are in the technology and financials space, with Apple, online retailer Ebay and Warren Buffett’s multinational holding company Berkshire Hathaway among its top-10 holdings.