Goldman Sachs is warning that this year's commodity price rally may be running out of steam, so investors should tread carefully and monitor data before going long oil and metals.

Commodities have bounced 12 percent this year after a steep sell-off in the final months of 2018. At this point, Goldman analysts say fundamental supply and demand will have to drive further gains — and they're not yet sure whether the figures will underwrite a further rally.

Goldman acknowledges that the market has moved past temporary drags like the longest-ever U.S. government shutdown, while China is embarking on a more expansionist policy. But the bank is still preaching caution.

"While this looks like it would point to even more upside for commodities, we believe that commodities have now reached a level where they are no longer signiﬁcantly undervalued relative to their current fundamentals," the investment bank's commodity analysts said in a research note Monday.

"The risk-reward of being outright long commodities is therefore less compelling now compared to a few months ago, and we recommend a neutral portfolio position in commodities."

In the oil market, Goldman believes demand is holding up despite gloomy forecasts. On the supply side, the bank says Saudi Arabia is taking a "shock and awe" approach to cutting output, while production in Venezuela and Iran is bound to fall further as the two OPEC members remain under U.S. sanctions.

That could push Brent crude oil futures toward $70-$75 in the near term, but Goldman sees prices slipping in the second half of 2019 on an anticipated increase in output from OPEC countries and U.S. drillers.