Barclays Global Investors is temporarily halting issuing new creation units for the iShares S&P GSCI Commodity Indexed Trust (NYSEArca: GSG), according to a Securities and Exchange Commission filing.

The exchange-traded fund currently has slightly more than $1.4 billion in net assets and acts much like another ETF-like fund, the U.S. Natural Gas Fund (NYSEArca: UNG) in that it invests in a pool of commodities. As such, GSG isn’t regulated like traditional open-end funds.

“Neither the Trust nor the Investing Pool is an investment company registered under the Investment Company Act. Shares of the trust are not subject to the same regulatory requirements as mutual funds,” said iShares in a new warning on its Web site about GSG.

The different set of regulatory approvals needed by commodity pools includes government approval to issue more shares at certain predetermined periods. This spring, UNG experienced a sudden jolt in assets as natural gas investors began betting on that sector of the energy market as a prime turnaround candidate given improved economic conditions.

At the time, UNG’s manager expressed frustration at the fund being forced to go back several times this year to get a green light from regulators to issue more shares. While it waited, investors found the open-end fund they’d bought began trading much like closed-end funds.

Even though UNG has been given approval to issue new creation units again, its managers so far have decided the prolonged delay of its third request in the past year—first made in May—makes the process too fraught with indecision.

In recent weeks, the SEC and other authorities have started to consider placing limits on how much funds can invest in any one segment of the commodities market. That has led to other ETFs and ETNs focused on commodities to also temporarily shut its creation unit process, effectively cutting off new investments into the fund and producing closed-end types of operating characteristics.

Now, GSG is the latest. On Friday, Barclays Capital announced it was temporarily halting the issuance of new shares in the iPath Dow Jones-AIG Natural Gas ETN (NYSEArca: GAZ). That followed a similar action by the PowerShares DB Crude Oil Double Long ETN (NYSEArca: DXO).

Others are sure to follow, if for no other reason than that indexes used by all four are tracked by other ETFs and ETNs. For example, there’s an iPath (NYSEArca: GSP) that also follow the S&P GSCI Commodity Index. Another ETN issued by Goldman Sachs (NYSEArca: GSC) provides similar coverage, yet in a more modified fashion.

You can read the filing here. (A link to the filing, dated Aug. 24, is also posted on the company's iShares site.)