You can file a US trademark application, and have a filing basis of a foreign trademark application, under section 44(d). In this case, the application must have been filed within the previous six (6) months of the US application, must be the same trademark filed for the same goods and services. This is also known as a priority basis. Also, the foreign trademark should not yet be registered, otherwise you would use a section 44(e) filing basis for foreign registration, instead. Be prepared to provide a filing certificate and filing information for your foreign trademark application.
You can provide two filing bases for your mark, and those who claim a foreign filing basis under section 44(d) may also claim a 1(b) basis for intent to use. If the foreign application is invalid, or out of the 6-month period, the 44(d) basis can be removed and the mark can proceed on the basis of intent to use. But if the application is filed with only the 44(d) basis, then the whole application will fail if that single basis fails. Conversely, if the mark is allowed but has not yet been used in the USA, the mark may still register when the foreign registration is shown, and the 1(b) intent to use basis is dropped.
In out office, the foreign application is typically from an earlier application filed in Canada, in Europe or in China, and the trademark holder also hopes to export to the USA. If the trademark holder has already sold products in the USA, then a 1(a) basis is preferable, but specimens of use and dates of use in the USA must be filed with the application.